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 - before modification, the squelch would be extremely loose, and almost never go into the short tail mode. I changed a few resistors and caps to the appropriate band-specific values, and it began working like a regular Micor A/S board. The best thing would be to try and dig up one of the paging " Spectra-Tac Receiver" manuals that used to come with some of the paging stations. This one manual contained the tech docs for almost every band version RF/IF board, and the various versions of the corresponding A/S boards. If I recall correctly, there were three or four versions of the Spectra-Tac A/S boards - one for low band, one for VHF/UHF, and one for 800/900. If the values for the 70MHz mid-band version don't quite fix it entirely, simply compare the difference in values between the VHF and mid-band versions, and you should be able to calculate the appropriate values for low band accordingly. If you have trouble securing said manual, let me know and when I get back to my office, I'll see if I have an extra copy I can give you. Eric KE2D --- In [email protected], "kq2h" <k...@...> wrote: > > --- In [email protected], "Thomas Oliver" <tsoliver@> wrote: > > > Tom, the receiver and squelch appear to operate normally when using a service > monitor to generate signal. Noise floor issues can certainly be a problem, > although a Motrac receiver works fine on the same exact antenna. It's getting > blasted by the same noise... > > > > > I wonder if it is a noise floor problem. How does it act on a service > > monitor when it is not hooked to an antenna picking up high levels of noise? > > > > tom > > > > > > > [Original Message] > > > From: Adam Feuer <feuera@> > > > To: <[email protected]> > > > Date: 7/12/2009 7:30:11 AM > > > Subject: Re: [Repeater-Builder] Micor Bi-Level Squelch Circuit > > > > > > I am a very big fan of the Micor bi-level squelch. It's action is > > > second to none on my 440 and 900 system however, I too am having the > > > exact same problem on my 29..540 receiver in carrier squelch. I just > > > will not take anyone into the noise but rather near quieting signals > > > just "pop out" and disapear under the squelch. I have tried cleaning all > > > of the pins and swapping out the audio control module but nothing seems > > > to help. Interesting enough, this is NOT happening on my 6 meter Spectra > > > Tac! > > > > > > At the moment, I have placed my 29.540 Spectra Tac aside and replaced it > > > with a GE Mastr II auxiliary receiver on 10 meters. The results are > > > much better as pertains to the squelch, but this is not to start a Micor > > > vs. Mastr II comparison.....something is just not right on 10 meters > > > with a Spectra Tac receiver. If it can work on all of the other bands > > > (especially six meters!) it should work on 10. > > > > > > If anyone has any suggestions, perhaps some part changes on the audio > > > control module, they would be greatly appreciated! > > > > > > Adam N2ACF > > > > > > kq2h wrote: > > > > I've been impressed with the operation of the Micor bi-level squelch > > circuit based on the M7716 IC. I use Micor Spectra-Tac receivers > > exclusively on 440, 902, 6 Meters and 222 MHz (VHF to 222 conversion) with > > excellent results. However, I am perplexed by 10 Meters. The Spectra-Tac > > squelch circuit doesn't work well at all. It squelches up on fading signals > > that aren't all that noisy and drops out for no apparant reason. It appears > > that the squelch is set too high, but it isn't. I've tried different > > audio/squelch modules, receiver boards and housings. Even the IMTS > > modification which defeats the short squelch tail doesn't help the > > situation. When a UHF reciever is substituted in place of the 10 Meter > > receiver, the squelch circuit works fine. > > > > > > > > An old 25-30 MHz Motrac receiver, with its simple 3 transistor squelch > > circuit, works perfectly on 10 Meters. There are no problems with drop > > outs. The receiver squelch remains open well into the noise. This leads me > > to believe that the Micor bi-level circuit needs to be optimized for 10 > > Meters. > > > > > > > > Does anyone know of any component value changes in the Micor bi-level > > circuit to optimize operation on 10 Meters? > > > > > > > > > > > > > > > > ------------------------------------ > > > > > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > ------------------------------------ > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > >
