I wasn't quite sure where to post this. I hope it's not too far OT ... but I figure it is of most interest to those using Xtal sets ...
I very often seem to use my RF generator and Icom HF radio ( mostly the digital frequency display ) to figure out what frequency I have a Xtal or TRF radio tuned to, or to scope out the range of a new tank circuit in a radio.
I know many of you also use spotter radios when doing DX work ... especially during contests ... and sometimes can't find a station with the spotter radio that you are hearing with your XTal set
So it came to me ... why not build something that provides an easy way to determine the frequency of an uncalibrated radio ... and this project started to formulate ...
Here's what the front panel would potentially look like ...
The spotter generates a low power RF signal, that can range from 100kHz to 10MHz, and is either CW or tone modulated ... the best part is that there is a digital frequency display of the frequency generated.
When you need to spot a frequency, just tap the MOD button and sweep the frequency adjustment until the tone shows up on your radio ... then read the frequency from the spotter. The CW mode can provide a BFO function when working beacons or SSB frequencies ... or perhaps offer a little exalted carrier operation when desired.
The back panel of the spotter would have these components ...
The RF output and power switch need little explanation. The Hi Frq
and Lo Frq
adjustments let you set the range of the frequency control on the front panel ... no need to sweep the whole 100kHz-10MHz range if you are only going to work the broadcast band. So the setpoints spread out the frequency control operation ... though I would still incorporate a 10 turn pot for the main frequency control ! The Level
control sets the output level of the RF signal. This would depend on what method you choose to couple the signal to your working radio. The Mod Frq
control lets you choose the modulation frequency of the tone used to modulate the spotter's carrier. This is one area that many would like to have their druthers on.
Here is a rough functional diagram of how the spotter could be achieved...
The frequency counter function is accomplished with a crystal controlled PIC microprocessor. I included the rather old-fashioned LED segment drivers for two reasons ... the drivers reduce the number of controlling PIC pins from 28 to 16 .. doable for a single device. The second reason is that I DO NOT want to use multiplexing for the frequency display. The spotter will be coupled to radios that are straining the limits to pick out weak stations ... the last thing you want near your radio is a device that is generating hash in the kHz ranges you are working in !!
The second PIC will watch the pushbuttons and provide the necessary signals to enable the VCO output, and provide modulation ... this one is a bit more nebuleous, as it depends on what is used for the VCO.
The VCO is the wild card at this point. I have used A CD4046 for hetrodyne bat detectors, and it is a nice chip to work with ... but it's range cuts off around 1400 kHz. I have found the Maxim Max038 ... which looks good - but requires a split 5 volt supply ... this could lead to the use of either two batteries, or a switching supply for the negative rail ( a no-no ). So, I am open to ideas for a circuit that can generate a 100kHz to 10MHz signal, with very few components, and allow for frequency control using just pots ( A VCO ).
This is by no means a finished project ... heck, the doodles on the drawing board are still in pencil
I'd love to hear any and all ideas that you might want to throw into the hopper ... circuit tidbits ... more controls / adjustments ... does this even seem to be something that anyone besides me would want at hand ???
The forum is back to you !!