Overload - RF or IF overloading first?

[nanZor]

I've had my share of "hot" GRE's, both GRE and RS badged and while the 800mhz band often required attenuation to control overload, I am wondering if anyone analyzed if it is REALLY the front-end RF amplifiers overloading, or the IF 455khz (or levels going into the detectors) that is overloading first?

My first experience with the IF or levels going into the detectors running very hot was with the Grundig 750 shortwave radio, which required padding resistors prior to the detectors to tame what *appeared* to be front-end rf overload. Of course using the RF attenuators helped tame the detector level overload, but that is a total sledgehammer approach in my mind. (review and pics of mod here on RR)

My thought is that while being festooned with external antenna jacks, it is most likely that most end-users of the 750 are going to be using nothing more than the 4 foot whip and having a high level of detector input could raise the *apparent* sensitivity level.

Could the same be happening here, assuming that most users are going to use nothing more than the oem-supplied duck that is more or less resonant on 144mhz and a very high level of detector input is used to compensate - and using front-end rf attenuation when all that is needed is lowering of the detector input levels instead?

I don't have the gear or skills to work on these things, but I am curious if this might be the case.

 

[Boatanchor]

Many of the problems with these scanners relate to the lack of bandpass filtering near the antenna port and the use of cheap SAW filters in the higher frequency IF stages. SAW filters have fairly poor filter characteristics, which limit the attenuation of adjacent channel and even strong 'in band' signals.

These scanners were designed to a cost. They really weren't intended or designed to have anything other than a telescopic whip or a sub-standard mobile antenna connected to the rear BNC socket.

Once multiple signals being applied to the antenna port are > -60dBm (easily achieved with an elevated base or high performance mobile antenna), these signals can bleed through the SAW filters and start to produce other internal spurious signals through mixing and IMD.

If you are in an area with multiple strong signals from, say nearby trunking sites, FM broadcast etc etc and you use a high performance antenna such as a discone or similar, you are asking for trouble.
If you need to hit the attenuator button, just so you can close the squelch or 'clear up a signal', you know that you have a serious problem.

Having said all that, certain scanners suffer much less from this problem.

 

[nanZor]

Agree on all aspects, although I didn't know about SAW filters. I had assumed that in scanners we are dealing with mechanical / ceramic filters, but I guess I need to read up on SAW filters a bit more.

Hmm... wonder if a user with smd soldering skills could replace a saw filter with a higher quality one from Mouser, Digikey or elsewhere - or replace them with ceramics possibly?

 

[Boatanchor]

You'll find that most, if not all scanners, have a relatively high first IF frequency to eliminate potential IF image problems caused by the scanners wide bandwidth front end filtering. Typically, this 1st IF is up around 250Mhz or higher. Crystal filters can't be made at these frequencies and this is where SAW filters come in. SAW filters can be made up to UHF frequencies, but compared to crystal filters, they have relatively broad bandpass characteristics and poor rejection outside of the passband. SAW filters are also relatively cheap, very small and are well suited to miniaturized surface mount construction in handheld and mobile scanners.

SAW filters offer little, if any protection to very strong adjacent channel signals and only limited 'in-band' rejection. Basically, any strong signals in the neighborhood of your desired signal, will make it's way through the 1st IF SAW filter to the 2nd IF stages.

Multiple closely spaced signals in high density trunking systems, are the worst offenders. You might have 6-40 voice/traffic channels, all within a couple of hundred Khz of bandwidth coming from trunk sites only a few miles away. The narrower the channel steps the worse the problem becomes.
12.5Khz channel steps were the norm for a while, now 6.25Khz channeling is becoming more commonplace.

All of these channels/signals together with the control channels may be >-65dBm or more and all of them are making their way through the first line of filter defense (the SAW filter).

If the unwanted signals are strong enough, intermodulation mixing starts to occur within the scanner and a large number of mixing signals can then be generated. Any one of these intermod products has the potential to interfere with subsequent IF stages and reception quality. P25 reception in particular, suffers severely, if there is any co-channel interference from internally generated intermod products from strong, nearby signals.

The best way to prevent all this from occurring is to have a relatively low IF frequency for the first IF, where narrow bandwidth, crystal 'roofing' filters can be employed. Most high end LMR radios employ a first IF below 60Mhz, a narrow (7.5-15Khz wide) 1st IF (roofing) filter as well as actively tuned, High Q front end filtering that eliminates most of the above problems. This is one of the reasons why high end LMR radios cost $2000 and a digital scanner only costs $500.