Comments from John Thorpe
             
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Intermodulation testing of high performance receivers
- by John Thorpe
Some thoughts on test methods and possible pitfalls... 
Essentially the problem is two-fold - firstly to generate a two-tone signal with 
as little distortion as possible, and secondly to establish that the measured 
results apply to the receiver under test and not to a combination of receiver 
and test equipment. 
Addressing two-tone signal generation first, a good hybrid combiner (eg 
Mini-Circuits PSC2-1) is blameless at signal levels up to 0dBm. Its IP3 is in 
the region of +50 to +60dBm (the exact value depends on frequency and port 
impedances) so it will not give problems until receivers get up to +45dBm IP3 
points. The biggest culprit producing IMD is the ALC loops in the output stages 
of the signal generators. Each generator detects some of the signal from the 
other generator as well as its own and the resultant beat frequency is amplified 
and applied to the output signal as AM. Sadly the de-facto standard, low-noise 
generator, the HP8640 is very prone to this and has an ALC bandwidth extending 
to several hundred kHz. (From what other people have reported most HP generators 
are poor in this respect, with R&S and Marconi Instruments products fairing 
better. Sadly intermodulation resistance to reverse power is not a specified 
parameter.) The amount of IMD produced by the generator ALC is dependent on the 
generator to generator coupling which in turn is dependent on hybrid coupler 
isolation and, more importantly, DUT input impedance (power is reflected if VSWR 
is not unity). I think that differences in antenna input VSWR is one of the 
causes of measurement differences between different high performance radios on 
suspect test equipment. (This implies that the argument "Radio X measures at IP3 
= +20dBm, so a measurement on radio Y of +15dBm is correct." is not valid). 
I have used three methods to improve the IMD performance of the two-tone signal 
source :- 
1) Replace one or both signal sources with crystal oscillators followed by 
grounded gate FET buffer amplifiers giving +10dBm output, which is then 
attenuated to the desired level. 
2) Add a high gain amplifier and then an attenuator between the signal generator 
and the combiner unit. The amplifier should be operated well within its linear 
range (maybe >10dB below rated output). I have successfully used +40dB 
amplifiers followed by 20dB of attenuation. 
3) Modify the ALC system of the signal generator sources. 
Any one of the above should improve the quality of signal fed into the receiver. 
With modified HP8640s I can achieve IM products more than 80dB below two signals 
each at +6dBm (verified with spectrum analyser). The crystal oscillators give a 
few dB better on a good day. Taking -90dB as a realistic target for the purity 
of the two-tone source means that any tests should aim to produce 
intermodulation in the receiver under test at least 10dB above this level for 
confident results. With signal injection to the receiver at 0dBm this sets a 
test equipment limitation of performance to an IP3 at +40dBm - good enough at 
the moment. 
Now to the actual testing. A quick thought will confirm that the higher the 
signal level the receiver is tested at, the less critical is the quality of the 
source. Testing at the noise floor level of the receiver will place unrealistic 
demands on the signal source if the receiver performance is good. I use receiver 
S-Meter readings to measure IM products at higher levels, so a test procedure 
goes something like this :- 
1) Connect signal sources to combiner and adjust to give two signals each at 
0dBm at the combiner output. 
2) Feed the two-tone signal through a 1dB-step attenuator to the receiver input 
and tune the receiver to an IM product 
3) Adjust attenuation until an S-Meter reading is obtained corresponding to a 
signal at about -80dBm (this should be about S9 on most receivers). 
4) Measure the IM product signal level accurately by substituting a single 
signal at the receiver input and adjusting for a similar S-Meter reading. 
5) The test can be repeated several times for slightly different two-tone signal 
levels. 

IP3 can be calculated from the measured results as :- 
<IP3> = ( <two-tone input level> - <single signal input level> ) / 2 + <two-tone 
input level> 
(all levels in dBm) 
Dynamic range can then be calculated by reference to the receiver's noise floor 
:- 
<IFDR> = ( <IP3> - <noise floor> ) * 2 / 3 
It is worth being aware of the pitfalls in this test method. To date I have only 
thought of one, which is that some receivers may start to invoke RF gain 
reduction from AGC action at the levels tested. This will certainly show up as 
an anomaly in the results (see later), but so far I have not met a "good" 
receiver that starts this effect below signals of S9+10dB. The AR 7030 does not 
add RF attenuation until S9+40dB. 
The most important facility that this test allows is verification. Because a 
range of input levels can be used the connection between two-tone input level 
and IM product level can be evaluated and compared with the theoretical 1dB / 
3dB relationship. If the test results can match this over several dB of input 
levels then one can be fairly sure that the IM products measured are developed 
in the DUT. 
As an example I have obtained the following results from a sample of the AOR 
AR7030 receiver at 100 kHz tone spacing (noise floor at -123dBm) :- 
      Two-tone
      input levelEquivalent IMP
      input level Calculated IP3 Calculated IFDR
      0dBm -68dBm +34dBm104.7dB
      -1dBm-72dBm+34.5dBm105dB
      -2dBm -74dBm+34dBm104.7dB
      -3dBm -78dBm +34.5dBm105dB
      -4dBm-80dBm+34dBm104.7dB 
      -5dBm-84dBm+34.5dBm 105dB
      -6dBm-87dBm+34.5dBm105dB 

	 	IM products rising at less than the 1dB / 3dB rate would indicate reception of a 
spurious response (rather than an IM product) or reception of a distortion 
product from the test equipment. Products rising at higher rates or rising 
erratically merit further investigation, often plotting a graph of input level 
vs product level is useful to attempt a straight line (of slope 3) through a 
scatter of points. Such effects seem to occur when several stages in the 
receiver generate IM products at more or less the same level, or when AGC action 
invokes a PIN diode RF attenuator (which usually increases IMPs). Predicting an 
IP3 value from erratic results has to be a matter of judgement! 
It is interesting to note that when similar levels of IM products are produced 
in the receiver and the test setup, or in several different stages in the 
receiver, that the signals may add (producing lower IP3 results) or cancel 
(producing higher IP3 s). Cancellation usually only occurs over a narrow 
frequency range (maybe only at one receiver frequency or one particular tone 
separation) but can produce bizarre results 10 or 20dB in excess of proper IP3 
values. 
John Thorpe March 1996, designer of the AR7030. e-mail: jt@aoruk.com 
Top



The AR7030, Intermodulation and Radio Netherlands 
- by John Thorpe 
(This has been revised in December 1996 following a further posting on the R.N. 
WEB site)
Intermodulation :- 
I had thought, and indeed hoped, that the receiver intermodulation measurement 
debate had been finally laid to rest but it seems that Radio Netherlands have 
exhumed the corpse ready for a second post-mortem. So, donning appropriate 
rubber gloves and apron, here goes.
Radio Netherlands keep moving their pages around.... For the RN text on tests & 
measurements click here
http://www.rnw.nl/realradio/rx_testing.html
For the RN AR7030 full review click here
http://www.rnw.nl/realradio/ar7030.html
The Idea of using third-order intercept point (IP3) and Intermodulation-free 
dynamic range (IFDR) as performance specifiers for a receiver is that different 
receivers can be compared like for like with regard to their strong signal 
handling capabilities. Given that test conditions are similar (frequency, signal 
separation, receiver filter bandwidth etc) then figures should be directly 
comparable. RN now introduce a further complication - how the measurement was 
done and indeed publish four different results in their latest review of the 
AR7030 (December 1996). It is not as if their results were broadly similar - 
they obtain IP3 figures from +1dBm to +33dBm, a difference of more than three 
orders of magnitude, in receiver terms ranging from a mid-price portable to a 
very expensive professional rack-mount receiver. They cover this anomaly by 
saying that "These values ... show that when you compare published receiver 
intercept points it is essential that you know the level of the [test] signals 
otherwise the IP3 figure is meaningless." 
Is the IP3 figure such a difficult animal to quantise, depending on external 
conditions ? Well, no, of course it isn't, and it's been used as a reliable 
indicator of circuit performance for many years in the radio engineering 
industry. So who is correct then ? Well I think that I am, and I can prove it 
(but then I would say that wouldn't I ?). 
Mathematical interlude :- 
On the basis that every equation will halve the readership of an article (S. W. 
Hawking) this is kept as short as possible. 
The transfer characteristic of a circuit, which describes its output in terms of 
its input, can be represented as a polynomial equation. A perfectly linear 
circuit would have zero coefficients for all polynomial terms above first order 
- it would produce no distortion. A real circuit has non-zero higher order terms 
which means that the signal passing through it is distorted to some degree. In 
considering the third order distortions the critical part of the equation is the 
cubic term. This means that the level of distortion is proportional to the cube 
of the input signal level, in other words if the input increases by 10dB then 
the third order products at the output would increase by 30dB. 

This input / output relationship is fundamental to the concept of intercept 
point (which is a theoretical point at which the intermodulation products are at 
the same level as the signals generating them). 
Proof 1 (Confidence in the test equipment) :- 
Any intermodulation measurement requires that several (typically two) signals 
are injected into the unit under test (UUT) and then the levels of any resultant 
intermodulation products produced in the UUT are measured. The IMD levels are 
typically referred back to their equivalent input levels for calculation of IP3 
and IFDR. It is important to realise that possible sources of IM products extend 
outside the UUT into the signal sources and measurement equipment as well. For 
the purposes of this proof I will divide the test setup into three parts:- 
Part 1 is the signal source, consisting of oscillators or signal generators and 
a combining unit. 
Part 2 is a variable attenuator (to control the level of signals fed into the 
receiver). 
Part 3 is the receiver under test, with any associated audio analysis equipment. 

It is assumed that Part 2 (the attenuator) does not generate any intermodulation 
products, i.e. it is blameless. Also although the audio analysis equipment is 
lumped into Part 3 it is not implicated in IMD production. 
Both Part 1 and Part 3 are capable of producing intermodulation products. RN 
claim that their crystal oscillator signal sources are "combined in such away 
(sic) that they do not influence each other" but offer no evidence to support 
this statement (note 1). 
The signal levels in Part 1 (before the attenuator) are essentially constant, 
varying only slightly as the attenuator is changed with any mismatches into Part 
3. Any IMD produced in part 1 should therefore be at a more or less constant 
level. 
The signal levels in Part 3 vary with the attenuator setting. Any IMD produced 
in Part 3 should therefore follow the cubic rate law (as above) and change three 
times faster than the signal level. 
Here are the figures from the RN tests :- Measurement
      level Two-signal
      inputEquiv IMD
      level IFDR IP3
      1-40dBm-122dBm 82dB +1dBm
      2-29dBm-107dBm 88dB+10dBm
      3-10dBm-87dBm100dB+28dBm
      40dBm -66dBm 103dB+33dBm

Let's first assume that the signal sources are blameless, and all of the IMD is 
generated in the receiver :- 
      Measurement
      Two-signal
      level Equiv IMP
      input level IMD level
      wrt signalExpected
      IMD levelError
      1-40dBm -122dBm-82dB -146dB 64dB
      2-29dBm-107dBm -78dB -124dB46dB
      3-10dBm-87dBm -77dB -86dB 9dB
      40dBm-66dBm -66dB -66dB(note 2)

Let's now assume that the receiver is blameless, and all of the IMD is generated 
in the signal sources :- 
      Measurement
      level Two-signal
      level Equiv IMP
      input level IMD level
      wrt signalExpected
      IMD levelError
      1-40dBm -122dBm-82dB -82dB (note 3)
      2-29dBm-107dBm -78dB -82dB 4dB
      3-10dBm-87dBm -77dB -82dB 5dB
      40dBm-66dBm -66dB -82dB 16dB

A quick look at the last column in the above two tables indicates much lower 
error values in the second case. This means that the second hypothesis more 
closely fits the data - indeed if we disregard the fourth measurement (because 
at 0dBm input levels we know that the receiver is NOT blameless) then the error 
values are more or less within an expected +/- 2dB measurement error for this 
type of test (note 4). 
The conclusion of this proof is that the RN test source generates 
intermodulation products at 80dB (+/- 2dB) below the level of the wanted 
signals. Using their test method with this source will misrepresent the 
performance of any radio with an IFDR greater than 78dB. 
Proof 2 (How does this work then ?) :- 
The RN intermodulation results in the first table (above) indicate a receiver 
that improves (produces less distortion) as the signal levels get larger. If 
these measurements are to be believed then to explain this behaviour requires a 
circuit in the receiver whose distortion firstly increases with increasing 
signal level, but then decreases as signal levels increase further. It is quite 
a challenge to design a circuit that behaves like this, and although I could see 
that a class AB push-pull amplifier might have this characteristic, the class A 
stages in a receiver would be hard-pushed to mimic it. 
This second proof is not as conclusive as the first (since it invokes "not 
imaginable" as reason for non-existence), indeed some may query if it is a proof 
at all, but it does avoid a lot of figures, and some may find it a convincing 
reason to doubt what RN have published. 
That's all for now. 
John. 
Notes 
1) It is fallacious to think that a 6dB pad (fixed attenuator) ensures immunity 
against impedance mismatch. In fact any adverse effect is only reduced by 6dB 
because although the reflected power is attenuated by 12dB the source power has 
to be increased by 6dB to overcome the attenuator. 
2) The last measurement is the least taxing for the sources so this is used as 
the 0dB error reference point. 
3) The first measurement is the least taxing for the receiver so this is used as 
the 0dB error reference point. 
4) Errors mainly come from receiver s-meter resolution, attenuator accuracy and 
impedance mis-matches.
John Thorpe, designer of the AR7030. e-mail: jt@aoruk.com
Top



Intermodulation News - more IP3 thoughts and tests
- by John Thorpe
The AR7030 receiver has proved to be an excellent platform for investigation of 
3rd order intermodulation distortion. Firstly its performance is good enough to 
tax most of the commonly available test equipment and secondly its signal path 
has sufficiently few stages so that intermodulation from each one can be 
measured and isolated. 
Recent tests on the 7030 by the ARRL have shown up some strange intermodulation 
effects which prompted me to re-visit all of the IMD testing with an improved 
test setup and provide an explanation for the observed results. These particular 
anomalies are discussed in the last part of this article which is an unashamedly 
technical (but non mathematical) look at intermodulation within HF receivers, 
methods of intermodulation testing and components that generate IMD. 
This article concentrates on third order intermodulation although its comments 
and conclusions are equally appropriate to both even and odd-order distortions. 
Odd-order intermodulation (3rd order usually being the most prominent) is 
normally used as an assessment of a receiver's dynamic range because there is 
very little that can be done external to the receiver to improve it. Even-order 
distortions can generally be reduced by filtering the receiver's input 
(pre-selection) because at least one of the signals causing interfering 
intermodulation products is well separated in frequency from the wanted signal.
Intermodulation test setup. 
In a standard test situation intermodulation products are generated between two 
equal level signals separated by a specified frequency. Typically two signal 
sources (signal generators) are used with their outputs combined into the 
antenna input of the receiver. Additionally a single signal (at the frequency of 
the expected intermodulation product) is usually needed to calibrate the 
receiver so the level of the IMD product can be determined. 
The main difficulty in generating and combining the two signals for the test is 
to prevent intermodulation within the test equipment, or at least ensure that it 
is at a level below the expected IMD in the receiver under test. Because the 
7030 is very demanding in this respect, I have previously suggested that testing 
should be done at a sufficiently high level to ensure that receiver IMD will 
swamp any input from the test equipment. I still think that this is a valid 
approach, but wanted to improve my own testing capability to include assessment 
of receiver IMD at low signal levels. 
There are three main sources of intermodulation within the test setup :- 
1) Distortion in the combining network. A simple resistive combiner will not 
generate distortion but will also not provide much isolation between the two 
signal sources which, in fact, is a bigger problem. A hybrid combiner is more 
satisfactory, but contains broadband transformers which can suffer from core 
saturation. In practice a Mini-Circuits PSC2-1 has an IP3 in the region of +50 
to +60dBm (the exact value depends on frequency and port impedances) so it will 
not give problems until receivers get up to +45dBm IP3 points. It is, however, 
worth operating the combiner at the lowest possible signal levels, so there 
should be no signal attenuation between the combiner and the receiver's antenna 
input. 
2) Intermodulation in the output amplifiers of the signal sources (due to 
cross-coupling of the two signals). The higher the power of the amplifier the 
better, so signal generators capable of +19 or +20dBm output are recommended. An 
additional linear amplifier after the signal generator can work well, but the 
common "single device" type of broadband amplifiers do not seem to be very good 
at rejecting signal at the output, and there can be additional problems of 
increased broadband noise. In any case the test setup should try to minimise 
cross-coupling (see later). 
3) Unwanted modulation of the signals by the level control system within the 
signal generators. Obviously this only applies to test setups using levelled 
signal generators as sources, but beware - many crystal oscillators have ALC 
systems to ensure low phase noise and these are just as vulnerable. The effect 
is due to the signal generator striving to maintain a constant output level by 
rectifying its output signal and using a feedback system to drive an amplitude 
modulator. If a second signal is applied to the output, then the detector will 
produce a signal at the "beat" frequency of the two signals (ie at the 
difference frequency) and this signal will modulate the generators output. The 
frequency of the modulation sidebands will neatly coincide with the expected IMD 
product frequency that we are looking for in the receiver under test ! The 
reason I have separated this effect from intermodulation in (2) above is that it 
is linear wrt the second signal level fed into the generator's output whereas an 
intermodulation effect would change at a third order rate. It is therefore 
difficult to isolate this effect by changing signal levels and attenuations 
within the test setup. I have only managed to quantify it by taking measurements 
within the ALC loop of the signal generator. 
The distortion from all of the above effects can be minimised by ensuring that 
there is as much attenuation as possible between each signal source and the 
combiner. Many signal generators do not maximise output attenuation, preferring 
to run their output amplifiers at lower than maximum levels (for better harmonic 
performance) so it is usually preferable to run the generators at maximum output 
and use an external 1dB step attenuator to control the level into the combiner. 
In the case of the HP8640 generators that I use this gives between 20 and 40dB 
more generator-generator isolation than simply connecting the combiner directly. 

The ALC modulation problem is severe in the HP8640 (it certainly affects IMD 
results in receivers with IP3 above +15dBm) but can be virtually eliminated by a 
simple modification to restrict the bandwidth of the ALC system to about 1Hz. 
This mod removes the AM capability of the generator, so it has to be switchable 
(I can provide details if anyone wants). 
As well as generating the intermodulation test signal the test setup must be 
able to establish the level of intermodulation distortion that the receiver 
produces. It is normal to treat the receiver as a "black box" with an antenna 
input and an audio output and use only these signals (rather than inserting 
probes into the receiver circuit to inject or monitor at intermediate points). 
In order to use the receiver's audio output to assess its IMD performance it 
must be used in a "linear" reception mode where the output reflects what happens 
at the input. Typically for an HF receiver this would be one of the SSB modes or 
CW - the intermodulation products can be resolved at a frequency near to the 
centre of the receiver's passband, say 1kHz. 
There are three ways of determining the IMD product level in a "black box" 
receiver. All have limitations, and often a combination of methods is required 
to produce comprehensive and confident results :- 
1) Calibrated S-Meter. 
The audio output of the receiver is not used other than as an aural tuning aid, 
instead the level of the IMD product is noted on the receiver's S-meter display 
and then a single signal is fed into the receiver to give the same S-meter 
reading. This method is good provided that the resolution of the S-meter is good 
- the absolute calibration of the meter does not matter - and favours testing 
the receiver at higher signal levels where the IMD products are well clear of 
the noise floor. In fact it is often the only way to determine the higher levels 
of IMD products. Beware that some receivers (not the AR7030) start to reduce the 
gain of the RF stages when the S-meter gets to the S9 or S9+10dB region - this 
will produce erroneously high results for IP3. Testing in the S3 to S7 region is 
generally OK. 
2) Audio output S/N ratio.
The audio output is tuned to exactly 1kHz and analysed with a signal / noise 
ratio meter or SINAD meter. Accurate tuning and low drift in the receiver and 
signal sources is a requirement but is not normally a problem with synthesised 
equipment. It is possible to estimate S/N with an audio power meter but this 
must be done very carefully because readings can be compromised by receiver AGC 
action - switch the AGC off if possible but then beware of limiting in the IF or 
audio stages. The S/N ratio measurement is good for signals at or up to 20dB 
above the noise floor of the receiver and give a reading relative to this noise 
floor. The big problem is that the noise floor is often raised during the IMD 
testing either by reciprocal mixing in the receiver or by phase noise from the 
test generators and this gives an impression that IMD levels are lower than they 
actually are. The noise increase can be largely compensated for by continuing to 
feed the test signal closest in frequency to the IMD product whilst introducing 
the calibrating signal (at a level to give the same S/N ratio as the IMD 
product). This assumes that the majority of the noise or reciprocal mixing comes 
from the test signal nearest to the receiver's tuned frequency. This test method 
is also very susceptible to spurious responses in the receiver if using a S/N 
meter to analyse the output. Any heterodynes thus produced will be considered as 
part of the noise - so always listen to the output and if necessary change the 
test frequencies slightly to avoid spurii (it is often a problem with 50kHz 
spaced signals - try 45 or 55kHz instead). 
3) Audio signal level.
Using an AF spectrum analyser or selective voltmeter (wave analyser) the level 
of the IMD product can be measured at the audio output irrespective of the noise 
level on the audio. Indeed with a narrow filter in the analyser (say 10Hz) it is 
quite possible to measure levels that are well below the noise floor of the 
receiver. This technique can give good results over quite a range of signal 
levels, but initially the receiver needs calibrating with a single signal and 
the maximum measurable level determined. This will either be the onset of AGC 
gain reduction or, if AGC is switched off, the limiting level in the IF or AF 
stages. 
The purpose of the above ramblings and my equipment modifications is to be able 
to investigate the levels of IMD products in a 7030 receiver over as wide a 
range of signal levels as possible and to verify that the input / output ratio 
follows the expected 1:3 relationship. If this is indeed the case then the 
computed value of IP3 should be a constant for all input levels. 
The 7030 was tested with signals at 14.020 and 14.070MHz (50kHz spacing) in USB 
mode with the standard 2.2kHz filter selected. The IMD product was resolved at 
1kHz with the receiver tuned to 13.069MHz. For low level inputs (up to -8dBm) 
the AGC was turned off and the audio signal level measured using a spectrum 
analyser with a 10Hz bandwidth. For higher signal levels (-6 to 0dBm) the AGC 
was turned on and the S-meter readings used. With the AGC off the 7030 is linear 
with antenna inputs up to -90dBm. 
      Two-signal input
      level (each signal) Equivalent IMD
      product level(ref to signal)Calculated IP3
      0dBm -66dBm +33dBm
      -2dBm -73dBm +33.5dBm
      -4dBm -78dBm+33dBm 
      -6dBm -84dBm +33dBm
      -8dBm-89.5dBm +32.7dBm
      -10dBm-94.8dBm +32.4dBm
      -12dBm-100.5dBm+32.3dBm
      -15dBm-109.7dBm+32.4dBm
      -20dBm-124.6dBm +32.3dBm 

These test were done with the RF preamplifier switched off, but the test results 
repeat almost exactly with the preamplifier turned on and the input signal level 
reduced by 10dB. The results show that the 1:3 input / output ratio is 
accurately maintained over a 60dB range of output from levels below the noise 
floor to over S9. 
Intermodulation in HF receivers. 
From a radio designer's point of view every receiver front-end is a trade off 
between sensitivity and dynamic range - the trick being to balance the gain (and 
hence signal levels) in each stage with the bandwidth of signals that it is 
subject to. If filtering is done before enough gain stages then sensitivity 
suffers. If too much gain is applied before filtering then there is a greater 
possibility of overload and intermodulation. In a typical dual conversion HF 
receiver there are two stages of filtering that affect the dynamic range (and 
IMD performance) of the receiver discounting, for the moment, any RF stage 
filtering. 
Normally the dual conversion receiver up-converts the RF signal to the first IF 
which is typically between 40 and 80MHz. There the signal is filtered, usually 
with a crystal filter of 15 to 20kHz bandwidth, and amplified before being 
down-converted to the second IF which is normally below 2MHz, 455kHz being 
common. The receiver's main selective filtering is done in this second IF stage 
with bandwidths depending on reception mode - 2.2kHz is normal for SSB 
reception. The dual conversion arrangement is popular because it works well, 
providing good to excellent image rejection and good filter shape factors 
without using excessively expensive components. Tuning is also straightforward 
requiring only one tuneable local oscillator (for the up-conversion to the first 
IF). 
With a few exceptions most HF receivers have no selective RF stages (after all 
the up-conversion has diminished the image rejection circuit to a simple RF 
low-pass filter cutting off below the IF frequency) or include only switched 
sub-octave filtering which helps with even-order intermodulation but does 
nothing for odd-order effects. The signal losses through the filters are a 
problem in the sensitivity stakes anyway. This means that all of the receiver's 
circuit up to the first IF filter (called the roofing filter) sees the whole of, 
or at least a large part of, the RF signals coming from the antenna. After that 
filter the circuit only sees signals within about 20kHz of the tuned frequency 
up to the selective filter which then eliminates everything except the required 
audio bandwidth. 
By choosing IMD test frequency separations appropriately it is possible to 
establish the performance of each section of the receiver. Separations greater 
than 50kHz mean that the test signals will be blocked by the roofing filter and 
so this tests the RF stages, the up-conversion mixer and any IF amplifier before 
the roofing filter. It is common for there to be little improvement in IP3 
figures once the 50kHz separation is exceeded - this simply indicates the lack 
of any RF selectivity in a receiver. Testing at a 4 or 5kHz separation will 
allow both IMD test signals through the roofing filter virtually un-attenuated 
so the next stage of circuit up to the selective filter is tested. This includes 
the first IF amplifier, the down-conversion mixer and the first stage of the 
second IF amplifier. Because these circuits have some gain (often a significant 
amount) the IP3 figure will be lower than for the 50kHz test. At test signal 
separations of a few hundred Hz the whole of the receiver is tested (assuming a 
2kHz bandwidth) though the results of this test tends to reflect on the 
receiver's audio quality rather than its dynamic range. 
In my opinion many receivers have too much gain before the selective filter, and 
the down-conversion mixer can cause severe IMD products when listening to 
signals close to strong stations. I have tested some receivers that give good 
+15 to +20dBm IP3 performance at 50kHz spacing but very poor results in the 
region of -30dBm at 5kHz spacing. Needless to say the 7030 is not designed like 
that, and the IF amplifiers and down-conversion mixer will return an IP3 of 
+12dBm. The price paid, of course, is in sensitivity, but even with 10dB of RF 
pre-amplification the close in IP3 should be better than 0dBm. 
So what happens when the IMD test signal separation falls between the 5kHz and 
50kHz values? Well I would expect a gradual transition of values as more of the 
test signal passes through the roofing filter with narrower frequency 
separations. And up to a point that is what happens, but some strange happenings 
can be observed along the way 
Intermodulation in passive components. 
In April the ARRL carried out some IMD tests on the 7030 as part of their 
receiver review. They tested the receiver at 20kHz frequency spacing with the RF 
pre-amplifier switched on and obtained an IP3 result at around +12dBm which is 
some 12dB below the expected value. They had tested the receiver with an IMD 
product level at the noise floor, but because of the discrepancy with AOR's 
measurements continued the test at different signal levels and produced a graph 
of signal level vs. IMD product level. This showed an unusual characteristic; 
the IMD product level rose quickly from the noise floor by about 5dB for a 1dB 
input change and then stayed more or less constant for a further 10dB change. At 
higher input signal levels it then started to follow the expected 1:3 slope 
which corresponded to an IP3 of +25dBm (more or less to specification). I 
repeated their tests and found similar strange effects although I was not able 
to duplicate the results exactly.
Detailed investigation showed that the effect was very frequency dependent (it 
did not occur at all at frequency separations above 25kHz) and was often 
inconsistent, giving different results on different test runs. A gradual 
elimination of components that may cause the effect pointed to the roofing 
filter being the guilty party and indeed slight warming or cooling of that part 
caused significant changes in the IMD level virtually confirming it as the 
culprit. 
That IMD should occur in a crystal filter is not surprising. The design of the 
RF stages in the 7030 has already been changed after initial production because 
of IMD in the 1.7MHz high-pass / low-pass filter. In this case both inductors 
and capacitors were responsible for IMD products near the filter's cut-off 
frequency (where voltages and currents in the filter tend to be highest) and the 
problem was solved by specifying larger inductors and a different dielectric for 
the capacitors. The point is though that the IMD in these passive components 
behaved in the expected manner - the product level rose at 3 times the test 
signal level. 
Whatever is happening in the roofing filter cannot be described with a standard 
polynomial transfer function - indeed the input / output characteristic must 
actually change with the applied signal level and as such its IMD performance 
cannot be characterised with an IP3 figure. Since the filter is a mechanical 
device - energy is transferred from the input to the output by way of physical 
motion - the best mechanism I can think of for the observed behaviour is some 
kind of rattle. If anyone with more knowledge of filter technology wants to 
offer explanations I am happy to listen. 
To summarise the observations - the roofing filter produces IMD products when 
one or both of the test signals are in the transition band of the filter 
(between pass-band and stop-band) and the level of these products does not obey 
the expected 1:3 input / output level ratio. The roofing filter in the 7030 is a 
four-pole fundamental mode crystal filter with 15kHz bandwidth at 45MHz made up 
of two monolithic dipoles. Its input and output impedances are matched to 500 
ohms and it is driven from a heavily damped tank circuit. A 0dBm signal into the 
receiver's antenna input at the tuned frequency produces 510mV of IF signal at 
the input to the filter and 380mV at its output. 
Conclusions. 
The work on signal generator isolation has now produced an IMD test setup that 
is capable of measuring the 7030's performance down to and below the receiver's 
noise floor. 
The resultant tests show that for signal separations of 30kHz or more the 
receiver behaves in a "textbook" fashion with consistent IP3 results 
irrespective of the testing signal level or the test method used. 
There are some as yet unexplained anomalies in the IMD performance of the 
roofing filter. 
John Thorpe August 1997. e-mail: jt@aoruk.com
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Typical e-mail message about AR7030 IP3...
I've just purchased the AR7030 from one of your distributors in the States. I am 
quite pleased with its performance. Recently a review from Kiwa Electronics 
mentioning a problem with the high pass input filter (lower IP3 at 2 MHz) has 
generated some concerns on my part. My question is: has AOR corrected the 
problem in later production run ? My AOR s/n is xxxxxx. Your response is greatly 
appreciated.
This is really a question of keeping a perspective on the issue. So far only a 
few people have ever noted the "notch" in IP3 and one was using half a mile of 
wire aerial at the time. In the worst case IP3 is still in the order of +10 to 
+15 dBm across a fairly narrow range with +30 dBm elsewhere... the set is still 
as good as anything else comparably or marginally higher priced "out there" even 
with the SMD filter components.
As with all manufacturers AOR reserves the right to improve products where 
possible & cost effective and this is what is happening to the AR7030. The 
component type (not value) has been changed for the next production run with 
output arriving in about 6 weeks time.
It IS possible to modify existing equipment and AOR has a policy of "open 
information & support". If equipment arrives back here at AOR MANUFACTURING we 
will be happy to modify the filters for a nominal handling charge plus return 
carriage. Of course for overseas customers the shipping costs are restrictive. 
We are happy to release details for the modification but placing leaded 
components onto SMD pads is not an easy task unless you are used to handling / 
working with SMD each day... technically the change is straight forward.
At this stage I am not sure what the policy of international distributors will 
be.
The whole subject of IP3 has been an interesting debate but in reality unless 
you have found mixing problems (related more to IP2 than IP3 anyway) then don't 
worry about it... For most people BEFORE and AFTER "off air" testing would not 
reveal *any* difference what so ever without top quality test equipment being 
used as the measuring tool.
For your interest we keep detailed records of manufacture. Serial number xxxxxx 
was "born" on 24.7.96. The IP2 was +83dBm and IP3 +34dBm at our test frequency 
around 11 to 12 MHz. The initial filter cals were 2.1, 5.4, 6.4 & 9.5 kHz and 
full I.F. /AGC calibration records are held: 92, 60, 11, 18, 7, 8, 8, 29, 14, 22 
& 4.
The current production sets now offer consistent IP3 figures (within a couple of 
dB) from 500kHz to 30MHz.
The "notch" with the first batch of sets was due to both inductors and 
capacitors behaving non-linearly, so it is necessary to change all of the L-C 
filter components. For a field modification the SM components are replaced with 
leaded ones, but these need to be soldered to the SM pads on the PCB. The 
existing SM components are not re-used and are not glued to the PCB so there is 
no problem in removing them.
Access to the area of board involved is made easier by removing the right-hand 
case side, and then unsoldering Relay RL3 (this needs some care because of the 
PTH board). The relay should be replaced after the modifications are complete.
Filter change details :-
These are all leaded components fitted with leads cut short and soldered to the 
SM pads on the PCB. Inductors are mounted vertically.
Capacitors : New parts are Philips 630 series, Med-K ceramic plate (100V)
C22 1n8 10%
C23 1n0 10%
C24 1n8 10%
C25 2n2 10%
C26 3n9 10%
C27 2n2 10%
Inductors : New parts are Siemens B78108-T series (axial leaded)
L8 3.3uH 10%
L9 3.3uH 10%
L10 6.8uH 10%
L11 6.8uH 10%
I must emphasise that this is not an easy modification, and requires some skill 
with a soldering iron to do a good job.
Best regards and good luck, John. jt@aoruk.com
Modification information



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specification changes for product improvement without prior notice. The 
performance specification figures indicated are nominal values of production 
units. There may be some deviation from these values in individual units. 
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