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A few words about intermodulation dynamic range and roofing filters:
“Roofing filter” is the current buzzword in high-end radios. Just what does it
mean? Basically, a roofing filter is simply the first IF filter in a radio. It is usually
placed as close to the first mixer as possible in order to be effective. The term
“roofing” stems from the fact that it protects the rest of the radio following it from
out of the passband signals.
There are two basic designs for modern radios. There are radios with only ham
bands that use a first IF in the HF region, typically between 4 and 10 MHz, and
there are radios that have their first IF in the VHF region, well above 30 MHz.
The latter are usually called “Up Conversion” radios. There are advantages
either way, and lets examine some of them.
The Orion, K2 and Omni are like the first type. The Yaesu, Kenwood and Icom
radios are like the second.
The first IFs in the Orion, etc., are in HF region. These filters are easy to make
and have been available for many years. In the up conversion radios the first IF
is at VHF, somewhere in the 40 to 75 MHz region. All are crystal filters, either
discrete or monolithic types. Narrow bandwidth VHF filters have not been
available until recently, so the radios with VHF IFs typically use 10 to 20 kHz
wide filters.
The ability of a radio to ignore strong signals near the tuned frequency is greatly
enhanced by a roofing filter. Ideally, the final desired selectivity should be in the
first IF to protect the following high gain stages from strong out of band signals.
At the lower IFs it is possible to use filters as narrow as 250 Hz. At VHF it is not
yet possible to make practical filters that narrow. Three or four kHz is about as
narrow as they go in the VHF region.
Roofing filters and dynamic range:
Following the antenna connection, most radios have an LC bandpass filter. This
filter is usually as wide as an amateur band or even wider. So, the first mixer
may have tens or hundreds of signals at its input while you are trying to separate
out one signal for copy. The ability of the first mixer to handle these signals
without excessive intermodulation is a function of its circuit design. It does have
a limit, above which there is intermodulation which becomes stronger than the
noise floor of the radio. The difference between these two levels is known as the
dynamic range. This characteristic is generally measured with just two signals
of the same strength and some particular frequency spacing. For two signals
within the operating band, this is called the third order dynamic range. When the
signal spacing is much greater than the roofing filter bandwidth, the dynamic
range of the radio is determined by the first mixer and any other early stages.
Most high-end radios today have a dynamic range in the area of 95 to 105 dB for
signal spacing of 20 kHz or more. As the signal spacing decreases, at some
point they will fall into the passband of the roofing filter. They will then impinge
on the following second mixer and IF stages, which will create intermodulation at
much lower levels. Thus for closely spaced signals the receiver dynamic range
drops dramatically to maybe 60 or 70 dB. The transition width from the first
mixer dynamic range limit to the second mixer limit is determined by the
bandwidth of the roofing filter.
Let’s make some guesses as to the signal levels involved here. Assume we
have a radio with 100 dB dynamic range and a noise floor of –135 dBm for
signals spaced 20 kHz. This means there are two signals -- one off tune by 20
kHz and the second off tune by 40 kHz -- and they create a false response on the
tuned frequency when their level is –35 dBm, or 100 dB above the noise floor.
How strong is that? Well, S9 in a typical radio is –73 dBm, so these signals are
38 dB above S9. Any signals weaker that that will cause no problem for these or
wider signal spacings.
Now, suppose the dynamic range within the roofing filter bandwidth is only 70 dB
and the filter is 12 kHz wide (as it is in the FT-1000MP). Two signals spaced at 3
kHz or less will fall inside this filter, and if they are 70 – 135 = -65 dBm or
stronger, they will cause intermodulation signals in the passband. This is only S9
+8 dB per signal. In a radio contest it is possible to have several signals in the
+/- 6 kHz band around your tuned frequency which are stronger than S9 +8 dB,
and this is why we hear false signals under those conditions.
Narrowing the roofing filter has no effect on widely spaced signals, as the
intermodulation takes place earlier in the signal chain of the receiver. However, it
can improve the receiver performance for close-in signals. In the above
example, if we reduce the roofing filter bandwidth to 4 kHz, as the Inrad Roofing
Filter mod does, the widest separation which will cause a problem becomes 1
kHz instead of 3. This can reduce the interference substantially in crowded band
conditions. So it’s apparent that signals spaced at the roofing filter bandwidth
divided by 4 is the minimum spacing at which the dynamic range of the radio will
be improved. Shall we go as narrow as possible? Suppose we use a 250 Hz
roofing filter. Signal spacings down to 62.5 Hz will be improved. Doesn’t this
seem a bit close to operate next to your neighbor in a contest? I think the DX
station would have some difficulty trying to copy one of you and not the other.
What is reasonable? Maybe something which starts attenuating at signal
spacings of 100 Hz makes sense. This is a roofing filter with 400 Hz bandwidth.
The other advantage of making the filter a bit wider is that the insertion loss is not
as great. Insertion loss can reduce the sensitivity of the radio.
Is an 8 pole filter necessary? How does a 4 pole filter compare? One difference
between the two filters is insertion loss. For a 500 Hz filter this difference can be
a difference of about 5 dB for a 9 MHz filter. The receiver overall gain should be
kept fairly constant as filter bandwidths change to preserve the agc
characteristics and to keep the S meter reading constant. Also, the receiver
noise floor can suffer if there is a gain reduction close to the front end. We need
to insert an amplifier or otherwise change the gain to make up for the extra filter
loss when a narrow 8 pole filter is selected. This can reduce the dynamic range
of the radio, as has been seen in the Orion performance numbers in the ARRL
review. So 4 pole filters have an advantage, particularly for narrow bandwidths,
even though the selectivity is not as good for signals falling down the skirts.
There is less advantage in going to a wider filter such as a 2400 Hz bandwidth.
For example, Inrad’s 10 pole, 2400 Hz filter has an insertion loss of about 2.2 dB,
while the 4 pole filter with the same bandwidth has a loss of 1 dB. The difference
of 1.2 dB is small enough that it could be ignored and the 10 pole filter would
provide better off-channel rejection. Thus for the SSB bandwidths a good 8 or 10
pole filter will outperform a 4 pole filter, but for the narrow bandwidths the simpler
filter is best.
Roofing filter products:
Ten Tec Orion: Some time ago, after much discussion with W4ZV, we
introduced a 4 pole, 600 Hz bandwidth filter kit centered at 9001.5 kHz for the
Orion. This filter was centered on the SSB frequency so it could be put into the
1.8 or 1.0 kHz slots in the Orion, allowing the use of a narrow roofing filter without
needing the extra amplifier which is switched in when selecting 500 or 250 Hz
bandwidths. As a result the dynamic range for close in signals is dramatically
improved.
Here is what Rob Sherwood said about this filter compared to the Ten Tec 1.0
unit. “At 2 kHz spacing, the improvement (in dynamic range) is on the order of 6
to 7 dB. At 1 kHz spacing the improvement is closer to 10 dB.”
We have discontinued the kit (it was our reference #352) in favor of a completely
assembled unit which is ready to plug in and it is our reference #762. (See our
Ten Tec page.)
Yaesu FT-1000MP series: Earlier this year we located a source which can
produce good filters at 70.455 MHz and other VHF frequencies. We procured
some and ran numerous tests. Initial results were promising and a mod board
was designed before the Dayton Hamvention. A run of 100 units was completed
and we tested and documented each unit. Customers receiving these filters
were asked to report their findings. Some of those comments are reproduced
below.
“I have been doing A/B tests with the Orion and the FT1000MP in crowded large
interfering signal conditions. I must say the FT1000MP is doing a very ex cellent job - and
earlier tests clearly showed the superiority of the Orion in those kinds of conditions.”
The real eye-opener was when I tuned across my neighbor ham's signal, which was
pegging the S-meter at 80 over. I had been listening just below his frequency for awhile
and had not even realized he was on the air. “
“I have turned on the noise blanker in the presence of the local strong signals, and can
hear no products when they are tuned outside the passband of the filter.
I consider the price a real bargain. You disproved the prevailing theory that 70 MHz
roofing filters were impractical. Keep up the good work.”
“It appears that the rig is less touchy to big signals when tuning around the band with the
wide filter open. Looks like a keeper...”
About the noise blanker…”I do notice the shotgun effect I used to get from the NB is now
almost totally gone. The difference here was phenomenal!!”
“Without any attenuation and IPO on, the sub receiver of course still shows
the intermod etc - but the main RX is clean. So I am a very happy customer.”
In Europe….”initial subjective listening on 20M SSB with 10 to 20+ dB over 9 European
signals crowding the band indicates improved close-in strong signal
capabilities.”
During Field Day: “I could walk over to the other stations and hear the difference
between the stock FT-1000's and my Inrad equipped FT-1000. Does the roofing filter
make a difference.....YES! Would I do it again......YES! Does the Roofing Filter give me
an advantage? YES! PS: Don't tell my competition about your Roofing Filter, I need all
the advantages I can get.”
Testing was also done at an independent laboratory (not ours) and a sample of
the performance obtained with our mod installed is shown below.
Signal Spacing
Blocking DR
IMD DR
20 kHz
5 kHz
146 dB
130 dB
93 dB
89 dB
If these performance parameters are important to you, indications are that you
can save from $1500 to almost $9000 by keeping your FT 1000MP, installing this
mod and not buying a new radio.
Performance data for the original radio and as modified are shown in the plot
below. These curves are plots made by injecting a signal into the radio antenna
connector at 7 MHz and detecting the resulting signal at the output of the RF
board on 70.455 MHz.
The wider trace is the unmodified RF board response and the narrow trace is
after the Inrad mod is installed. There is a small increase in gain which has a
slight effect on lowering the noise floor.
1. What can you expect from this mod?
Less IMD in crowded band conditions, particularly from stations at offset
frequencies of 2 to 10 kHz either side of the operating frequency.
2. Will it defeat the noise blanker?
No, the roofing filter is in the circuit before the noise blanker sample is taken.
The filter delay is added to both signal and noise.
3. Will this mod allow for wide band SSB?
No, the roofing filter will determine the overall widest bandwidth of the receiver,
which is about 4 kHz wide. It will also limit the bandwidth of AM signals when
both sidebands are being detected. The sub receiver is not altered and can still
be used for the wider bandwidth modes.
4. Why not make the bandwidth narrower for CW use?
It is quite difficult to make narrow filters at 70 MHz. There is quite a bit of
variation from unit to unit, so we have chosen a bandwidth that will work for SSB
and CW even with the worst-case variation in bandwidth.
5. If 6 poles work so well, why not 8 poles?
The most important part of the filter characteristic is from the pass-band on down
to about –30 dB on either side of center. Eight poles would provide much better
stop-band isolation, but it’s not required in a roofing filter and would make no
noticeable improvement in IMD performance.
6. Who can benefit from installing this mod?
Operators who experience lots of strong signals which are close together in
frequency. This would include contesters with big antennas, people with really
good antennas and a good location, and dxpeditioners who get large pileups.
Casual operators wo uld probably not notice any difference in receiver
performance.
Ten Tec Omni VI series: We are presently working on a roofing filter mod for
the Omni VI+ radios. Plans are not quite formalized yet, but some preliminary
data from our lab is shown below.
Signal Spacing
IMD DR
Filter Bandwidth
20 kHz
5 kHz
100 dB
93 dB
2800 Hz
2800 Hz
20 kHz
5 kHz
1 kHz
100 dB
100 dB
90 dB
400 Hz
400 Hz
400 Hz
We hope to make this unit available in the late fall or winter of this year.
IC-765 and IC-781: Sample filters for feasibility testing are on order and will be
here in August. Stay tuned……
Other radios: As time permits.
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