Want to add frequency display to QRP transceiver

Thread Starter

upand_at_them

Joined May 15, 2010
621
I have this Pixie low-power transceiver that I want to add a frequency counter to. I have both units built, but I'm not sure where in the transceiver to tap into the oscillator without affecting it. The oscillator is built around Q1/Y1 in the schematic below. Do I use a low value cap to couple the signal to the frequency counter?

I guess this is the same as asking "Where can I probe the oscillator signal with my oscilloscope?" I can probe at the crystal and get the crystal frequency (or close), but anywhere else and I get different, and way off, readings.

Pixie schematic.jpg
 

jpanhalt

Joined Jan 18, 2008
10,918
Quite some time ago, I wondered about the actual internal frequency of an LCD. I made a simple loop antenna from coax and probed. A minimalist amplifier and my scope/Racal Dana gave me the frequency. If you measure your actual TX frequency with anything -- maybe a borrowed signal analyser or SW radio. Then once you know what it is, you can probe and find a spot where loading doesn't affect it. What frequency? You may still need to monitor the transmitted frequency.
 

Thread Starter

upand_at_them

Joined May 15, 2010
621
As in the circuit above I'm using a 7MHz-ish crystal (several crystals to try between 7.023-7.122). I do have a SW receiver, but it didn't occur to me to use it to check probe loading. Sounds good, I'll give it a try.

Coax might be too big for the size of this board. Can I do it with magnet wire?
 

jpanhalt

Joined Jan 18, 2008
10,918
As in the circuit above I'm using a 7MHz-ish crystal (several crystals to try between 7.023-7.122). I do have a SW receiver, but it didn't occur to me to use it to check probe loading. Sounds good, I'll give it a try.

Coax might be too big for the size of this board. Can I do it with magnet wire?
Probably. It won't take much to pick up the signal. You may want to match it to the impedance of your receiver, but also not necessary. You just don't want to overload the receiver's RF.
 

Thread Starter

upand_at_them

Joined May 15, 2010
621
No only do I have several crystals to swap in, but I intend to eventually build in more adjustability to the oscillator. I'm still wondering if there's a good point to tap into the oscillator signal...I'm working on adding a pre-amp for the frequency counter to minimize loading.
 

MrChips

Joined Oct 2, 2009
22,478
There is no good point in the circuit tap in. As John says, create a small loop antenna to pick up the radiated RF signal.
 

Thread Starter

upand_at_them

Joined May 15, 2010
621
Is there an ideal size of coil? I'm not getting a stable reading, and no closer than 8MHz (for the 7.05MHz crystal) using three turns of 22 gauge about the diameter of a pencil.
 
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