Adam,
Ten meter FM requires some different approaches mostly due to HF propagation
characteristics and less importantly, general HF background noise.
1) When sigs are propagated via the F2 layer (the most common kind of
ionospheric propagation, after sporadic E-layer), we all know they suffer from
fading and distortion. These occur because a signal takes many slightly
different paths on its journey to a distant receiver.
These paths take many different routes which results in various time delays
among them. They wind up really mangling, or helping, one another, depending
on their relative time delays which lead to various phase shifts among them.
E.g. 180 degrees of relative phase shift between 2 sigs = cancellation.
This multipath propagation is what causes selective fading (phase distortion).
The resulting distortion can get quite awful when using AM but really terrible
on FM, which is much more susceptable than AM to multipath.
You can see multipath in action on a tv set using a rabbit ear antenna: It's
called "ghosting". Also can hear it on a car FM broadcast receiver in built up
areas, as brief repeated bursts of noise and distortion as you drive.
2) We know that FM receivers use a noise-operated squelch circuit. It's
based on the fact that as FM sigs get stronger than the noise & distortion,
their output "quiets". Noise-operated squelch cts. take advantage of
this--they open when the signal is quieted by a certain, user-adjustable
amount. Note that FM quieting is proportional to signal to noise ratio, not
just signal strength alone.
However, we don't want our squelch to respond to any voice modulation, which
runs up to about 3-4 kHz. So, the squelch samples the discriminator (not
de-emphasized yet) output above 3-4 kHz (to about 6.5 kHz, in a 5 kHz deviation
system), where there _should_ be only some higher audio-frequency energy that
sounds like white-noise "hiss".
Here's where 1) and 2) come together: Multipath propagation of FM signals
during deviation peaks fills the audio spectrum _above_ 3-4 kHz with a TON of
noise and distortion. Now our sacred squelch area (3 to 6.5 kHz) is filled not
just with hiss but loads of modulation-related products.
Looked at closely, multipath distortion on an FM signal will make a signal
"appear" to be heavily over-deviated! (It does an analagous thing to AM sigs,
producing what sounds like (is, actually) over-modulation. Bummer.
The poor squelch, unless carefully designed, will close on deviation peaks,
even more than "talk down" or "squelch clamping" due to excessive high
modulating frequencies/high deviations will. Even an undeviated carrier under
conditions of multipath will show some extra distortion.
Due to the above, the Micor lovely dual-action squelch is seeing supersonic
(squelch spectrum) crud on just about all ionospherically propagated signals.
Have you noticed that ground wave signals do NOT have this problem with the
squelch?
It's conceivable that less hysteresis would help your situation. I'd also
explore moving the supersonic audio bandpass around some, keep the upper limit
the same but raise the low limit somewhat. Note that supersonic noise does not
extend much beyond about 1/2 of the IF bandpass. You could put an audio
spectrum analyzer at the output of the squelch noise amp, before rectification,
to see what freq's are present during various conditions, e.g. sigs with &
without multipath.
It would be interesting to sit down & look at the squelch noise amp bandpass
ct. of as many FM rx's as you can. It's usually quite simple, one or two
sections of high/lowpass RC circuits, sometimes a single RLC+C filter. Find
the -3 dB points. Perhaps the Micor's low end is set a bit too low.
It's a compromise between having sufficiently sensitive squelch action and no
squelch clamp down due to over-deviation & multipath.Tough choice.
BTW, the less the deviation, the harder it is to design reliable squelch
circuits. It was easier in the +-15 kHz deviation daze. The supersonic audio
spectrum will be different for other tx deviation/rx IF bandwidth situations.
For example, the olde Motorola Sensicon tube-type receivers (remember--18 tubes
& a big Permakay block IF filter) could be wired for either +-5 or +-15 kHz
use; most of the changes were in the squelch area.
(I converted a few myself.
)
--John WB0EQ/VE6
Alex,
You most likely need to change a few resistors and maybe a cap or two in the
squelch circuit of the SpectraTac's A/S card. The components will be near the
M7716 chip. If I recall correctly, some of the parts involve the chip's timing,
while others tailor the discriminator audio to compensate for the variations in
white noise between the bands.
A few years back I took a 70MHz SpectraTac chassis and replaced the 70MHz RF/IF
board with a 147MHz receiver, and I had the opposite effect - bef
