Screwball Selectable Squelch System

[leong]

Just bought an Icom M72. Very nice VHF, but:

All threshold settings of the squelch (1 - 10) suppress audio in the absence of a sufficiently strong signal (a setting of 0 turns off the squelch). Accordingly, I have no way to know whether a weak signal will come through even at the lowest setting. That is, I have no way to optimally choose the squelch level.

For a normally designed squelch the usual operation is to adjust the control (i.e. set a squelch threshold) until the receiver noise just shuts off - then any signal above the set threshold will be heard through the speaker. Also, if a weak signal is annoying, the operator can adjust the squelch to open only when stronger signals are received.

Can anyone make sense of the Icom M72 squelch system? Perhaps I've overlooked something. My guess is that other Icom models are similar with respect to the squelch adjustment.

[EEL]

[quote name='leong' date='Aug 5 2008, 11:20 PM' post='26758'

Can anyone make sense of the Icom M72 squelch system? Perhaps I've overlooked something. My guess is that other Icom models are similar with respect to the squelch adjustment.

I hear you about preset squelch threshold steps, but nearly all current radios work this way.

In a RF quiet environment the first step will quiet the radio and that is where to set the radio.

If you get in a noisy environment you can set it higher as needed. I would not worry about missing weak signals when on the first step since there is nothing you can do about it for starters.

FWIW some radios have more finely graduated steps or at least more steps. All my ham handhelds work this way and even one of my base/mobile rigs does too.

Ed Lawson

[leong]

[quote name='leong' date='Aug 5 2008, 11:20 PM' post='26758'

Can anyone make sense of the Icom M72 squelch system? Perhaps I've overlooked something. My guess is that other Icom models are similar with respect to the squelch adjustment.

I hear you about preset squelch threshold steps, but nearly all current radios work this way.

In a RF quiet environment the first step will quiet the radio and that is where to set the radio.

If you get in a noisy environment you can set it higher as needed. I would not worry about missing weak signals when on the first step since there is nothing you can do about it for starters.

FWIW some radios have more finely graduated steps or at least more steps. All my ham handhelds work this way and even one of my base/mobile rigs does too.

Ed Lawson

Thanks Ed,

I guess this is one downside of discrete (preset) squelch steps. But I wonder if the lowest squelch step is fixed or dynamically adjusts to the noise power that increases with circuit temperature? If fixed, then the first squelch threshold is probably set too high for cold weather operation. My other VHF, a Raycom 106 (about 10 years old, still works but needs a new NiCad battery), has a continuously adjustable squelch knob. I always set it just above the point where the noise is turned off. So, in effect, I can dynamically set the lowest squelch threshold.

The M72 has a "Monitor Function" which, as the manual says, temporarily releases the squelch to check the volume level. Perhaps another use is to temporarily see if any signal is being squelched out!

Do any of the radios with preset squelch levels have a way to set the lowest preset?

[EEL]

I guess this is one downside of discrete (preset) squelch steps.

Yes, but the upside is the sequlch does not get reset by accidental movement of the knob. Much like needing to press and hold a power button to turn radio on and off makes accidental operation less likely.

But I wonder if the lowest squelch step is fixed or dynamically adjusts to the noise power that increases with circuit temperature?

Interesting question, and I'll let those who do RF design for a day job answer, but my SWAG is the difference in internal noise due to temperature changes within rated operational range is not likely to be of practical importance. Since these are software defined radios, most anything is possible and maybe they do compensate for temp.

Do any of the radios with preset squelch levels have a way to set the lowest preset?

I am unaware of any that do. On my Standard 270 the lowest preset results in squelch being broken rather easily by junk so I suspect the lowest setting is pretty close to a practical minimum.

Ed Lawson

[leong]

Interesting question, and I'll let those who do RF design for a day job answer, but my SWAG is the difference in internal noise due to temperature changes within rated operational range is not likely to be of practical importance.

Ed Lawson

Perhaps I'm beating a dead horse, but going from 0 degrees F to 100 degrees F is about a 22% change in Kelvin temperature units. Thus the receivers thermal noise changes by this percentage (which is almost a dB). In addition, there is atmospheric noise, solar noise, man-made noise ... all dependent on time of day, weather, season, etc. Therefore, a fixed minimum preset squelch level might be too high for the weakest signals out there at a given time.

For the few times I’ve used my new M72 the lowest squelch level was sufficient to cut out the receiver noise. I still wonder if it also cuts out signals. I’ll feel more comfortable on a day when a higher squelch level (one of the other 9 presets) is needed to cut out the noise.

[jason]

Perhaps I'm beating a dead horse, but going from 0 degrees F to 100 degrees F is about a 22% change in Kelvin temperature units. Thus the receivers thermal noise changes by this percentage (which is almost a dB). In addition, there is atmospheric noise, solar noise, man-made noise ... all dependent on time of day, weather, season, etc. Therefore, a fixed minimum preset squelch level might be too high for the weakest signals out there at a given time.

For the few times I’ve used my new M72 the lowest squelch level was sufficient to cut out the receiver noise. I still wonder if it also cuts out signals. I’ll feel more comfortable on a day when a higher squelch level (one of the other 9 presets) is needed to cut out the noise.

The M72 has the standard squelch with the monitor button that you can adjust by holding the monitor and hitting the up/down keys. Along with the standard squelch it has a DSP squelch, that you have to access via the program mode. (On/Off) see page 21 of the manual.

The manual can be found at:

http://www.icomamerica.com/en/downloads/De...px?Category=114

-Jason

[EEL]

The M72 has the standard squelch with the monitor button that you can adjust by holding the monitor and hitting the up/down keys.

-Jason

Yes, but his concern is that the squelch adjustment is by discrete and pre-programmed steps and not a continuous manual adjustment as you get with a squelch knob and that in theory could result in even the lowest squelch setting (other than open) masking readable signals. As a user of the 72, you may be able to put his mind at ease regarding weak signal reception. The DSP option is intriguing...I wonder what that does...perhaps uses DSP for noise reduction to lower the thresholds for the squelch steps?

I suppose one practical test of this is to ascertain if there are distant WX stations that can be copied with the squelch open that are masked at the squelch set on the first step. Perhaps a little on water testing in order.

Ed Lawson

[jason]

Yes, but his concern is that the squelch adjustment is by discrete and pre-programmed steps and not a continuous manual adjustment as you get with a squelch knob and that in theory could result in even the lowest squelch setting (other than open) masking readable signals. As a user of the 72, you may be able to put his mind at ease regarding weak signal reception. The DSP option is intriguing...I wonder what that does...perhaps uses DSP for noise reduction to lower the thresholds for the squelch steps?

I suppose one practical test of this is to ascertain if there are distant WX stations that can be copied with the squelch open that are masked at the squelch set on the first step. Perhaps a little on water testing in order.

Ed Lawson

I will have to give it a test when I get it back, my radio M72 is in AK or on the way back from AK. I am using my Yaesu VX-170 for now.

-Jason

[leong]

I will have to give it a test when I get it back, my radio M72 is in AK or on the way back from AK. I am using my Yaesu VX-170 for now.

-Jason

By DSP squelch I assume you mean something like a Digital Signal Processor that measures the level of signal energy in various segments of frequency spectrum of a received signal to dynamically establish a squelch threshold.

But how do you know the M72 has a DSP squelch? The manual just calls it "Squelch Sensitivity Function" and says this about it "When this function is turned ON (local), blocking against noise is improved and the squelch is not easily affected by noise".

I don't think the Squelch Sensitivity Function addresses my concern. I'm afraid that not having an adjustable lowest threshold for the squelch may mute my signal if the available lowest squelch threshold is too high.

I turned the “Squelch Sensitivity Function†to ON. As far as I can tell it made no difference. The first level of squelch (still) mutes the receiver in the absence of a signal. My concern is that perhaps it also mutes the receiver when only weak signals are present.

I’d be a happy camper if the first one or two squelch levels didn't mute the receiver, but the next one did. Then, at least, I’d be certain that the first squelch level that did mute the receiver would be as close to the noise level as possible (within the quantification capability of the 10 available squelch levels).

[bob budd]

As far as the circuit performance, it is not difficult to design a reference that is relatively immune to ambient temperature. However, atmospheric noise as a function of time and/or temperature is a variable the squelch would not compensate for. Such behaviour would be expected to be roughly equivalent though with discrete steps you may fall between steps at times. As mentioned, the analog knob provides relatively infinite granularity with the potential for accidental disturbance of its position. Also, granularity should never be mistaken for accuracy or stability and often is an indication of poor accuracy and/or stability.

[leong]

As far as the circuit performance, it is not difficult to design a reference that is relatively immune to ambient temperature. However, atmospheric noise as a function of time and/or temperature is a variable the squelch would not compensate for. Such behaviour would be expected to be roughly equivalent though with discrete steps you may fall between steps at times. As mentioned, the analog knob provides relatively infinite granularity with the potential for accidental disturbance of its position. Also, granularity should never be mistaken for accuracy or stability and often is an indication of poor accuracy and/or stability.

Bob

Granularity is not the problem I'm addressing. If there is no way to set the threshold level that corresponds to the lowest squelch setting then usable signals may be muted out. That’s the potential problem as I see it.

[Bob C]

Squelch circuits described in this thread come in one of two flavors:

1. Analog implementation: a potentiometer sets the reference voltage for a comparator and the other input is the received-signal strength indication (RSSI), derived from either a multistage log/limiting amplifier (SA636 or similar) or from the limiter current in a cheaper discrete IF strip. The CARRIER LEVEL's (not the audio!) exceeding a preset threshold unmutes the audio. Adjustment of the squelch threshold is continuous via the potentiometer. The drawback is that potentiometers get dirty and can fail over time AND the shaft hs to somehow pass through a waterproof barrier.

2. Digital implementation: an ADC measures the RSSI signal and a microcontroller sets the mute threshold. When the carrier level is above a certain threshold the microcontroller unmutes the audio. Adjustment is in discrete steps with levels as defined by the programmer. These can use a push switch with a waterproof membrane between the outside world and the contacts to set the squelch by cycling through a few settings. These radios are still mostly analog; the microcontroller controls the synthesizer and the squelch. My IC88 still uses a potentiometer to set the audio level.

Neither solution uses DSP (digital signal processing); neither solution is a software-defined radio, which is a much more complicated and expensive implementation of a receiver.

In both cases the squelch is opened up by the carrier level (or signal strength), not the audio level.

Hope this helps!

Bob