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I have a wireless transceiver. It is programmable to allow both transmit and receive from 1-1000MHz with FSK or ASK. The PCB and components have been chosen for transmission at 433MHz. Another PCB variant is available, tuned for 915MHz.

If I attempt to receive 433MHz signals through the 915MHz tuned board or vice versa, how bad will it be? (I am not concerned with transmitting via a mistuned antenna).

How do I quantify this badness and what factors affect its magnitude?

Will signal quality reduce as well as transmission distance?

Will FSK perform better than ASK?

Can it be reasonably expected to work so long as the user "holds it a bit closer"?

Toby Jaffey
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2 Answers2

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There is no way to tell from the information you have given. How well a system works at one of 433 MHz or 915 MHz when "tuned" for the other can vary greatly. If this tuning (whatever that means) is highly resonant (high Q), then performance at any other frequency could be very bad.

If it is known up front that a system must operate well at both these frequencies, then that could be to reasonable levels of good enough. A antenna could be designed that is effective at both frequencies, although it won't be very selective on receiving.

Take a look at the electronic parts that are changed between the two versions. If these involve components of a resonant circuit, which they probably do, then only experimentation will tell you how well it will work off frequency.

I don't think there is any inherent advantage of ASK versus FSK other than the general features of each of these. Each will still be a relatively narrow band signal, so the spectrum won't be a issue.

As for signal "quality", I don't know what you mean by that. Ultimately everything is quantified by signal to noise ratio. A 433 MHz narrow band ASK signal is just that. There is really no measure of "quality" to apply other than ultimate demodulated signal to noise ratio. This signal to noise ratio is dependent in large part on received signal strength, but also dependent on other factors like received strength of other noise sources and amplitude above the inherent noise floor. Again, there are too many possible ways the circuits could work to give a meaningful answer from this simple outside view.

Toby Jaffey
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Olin Lathrop
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Hopefullythis will not be your best or only answer :-). This could be horrendously wrong - with RF and multiple interacting factors Murphy can visit (not Kortuk) but:

It's been a while since I've played RFing (as may show) and haven't tried in that sort of arrangement, but that's notionally only an octave away or 6dB/pole.

Again "notionally" reducing range from 50m to 2m is 25:1 so with inverse square loss, gain is (notionally) up 20 log(625) ~~~= 55 dB.

Allowing loss in rcv gain for PCB tuning and antenna tuning with VSWR horrendous I'd still expect it would tend to work. A piece of wire soldered across tuned portion on PCB may actually assist.

So "stand closer" may well work.

If I had to guess I'd say that FSK may have an edge, but again, only a guesstimate. (Detector looking at frequency and relatively amplitude unaware may be happier if system is sitting on a steep gain slope or has nice VSWR reflections. That produced a thought (possibly srurious) -light resistive loading may improve result by adding extra loss but potentially reducing chances of gross reflections producing a nasty result. Again, trying it is wise.

A Smith chart (or computerised implementation thereof) has a good chance of allowing you to model this well enough to be useful.

Trying it even in a semi-equivalent lashup would be wise.

Russell McMahon
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