4SQRP Stand Alone VXO Project


Kit assembled, tested and made operational by W5USJ
Assembly manual and instructions with additional help information is available from the 4SQRP website

RF Probe Etched PCB

The SAVXO, designed by Jim Kortge, K8IQY, is kitted and sold by 4SQRP

Mods and Projects

♣ 3-pin SIP strips as crystal sockets to facilitate band change along with modified crystal grounding
♣ Individual SIP pins for sockets at key frequency related component locations
♣ Driver for QRP full-gallon amplifers

Note: The following are documents/drawings/pictures describing the SAVXO mods and testing

♣ View/Download SIP strip crystal sockets
and SIP pins for band-change mods —SIP Strip Crystal Socket and band-change Mod PDF

♣ View/Download Crystal Ground Mod —Crystal Ground Modification PDF

♣ View/Download QRP amplifer Driver —QRP Amplifer Driver PDF


Initial Test Results
From a message to the 4SQRP group list...


• Did some additional testing and measuring using the 7122 nominal crystals supplied.
• Used my FT-847 zero beating with the 25 Hz filter for approximate frequency (checks good with WWV on 5 and 10 MHz)
• Power source is regulated and adjustable -- For testing set the supply voltage to 12.5
• RF RMS voltage is measured with a calibrated RF probe, fairly accurate

So, with my particular batch of parts and tools and their associated tolerances...

Frequency Ranges:
18 uH series choke supplied
~7111 to 7122 kHz
15 uH
~7119 to 7124 kHz
10 uH
~7122 to 7124 kHz

Power output set to ~250 mW, about mid way on trim pot. Measured ~5 Vrms across 100 ohm load, Po = Vrms^2 / Rload

With the trim pot set at max the signal apparently over drove Q3. The result was a near short on the power supply and a temporary loss of voltage.

At the ~250 mW level, the 100 ohm load got warm to the touch, about the same as the Q3 heat sink. Could only hold uncalibrated fingers on for a few seconds...8^)

Suggest using a well regulated stable power supply for the SAVXO. The frequency changes slightly with supply voltage changes. If also connected to the VXO, keying a transmitter from a poorly regulated supply could cause a chirp. A battery discharging over time would also change the frequency.

Now, to get a few more crystals from ESS and try band changes. Jim already did that for 17 meters with one of his early prototypes.

I'll probably try 30 meters first using 10125 kHz crystals and pulling them down around the 10116 QRP frequency.

This is fun stuff...


2012 Chuck Carpenter, W5USJ — Rev: 2 Mar 2012