10 MHz VXO: personal test and measurement about the SuperVXO
all rights reserved to I0SKK A. Santucci
A couple of years ago I tested a 16 MHz superVXO by JA0FAS and JH1FCZ and obtained a good result as frequency swing and spectrum analyzer report.
Now I'm planning to do a supererthet VLF receiver with an IF at 10.240 kHz, so I need a 10.240 kHz Local Oscillator, and stated to test a superVXO configuration, using some old FM RTX 10.240 kHz XTALs.
My tests were conducted expecially on waveform and frequency stability, as I need the VLF receiver in order to demodulate QRSS signals, so I need a rock steady L.O.
The frequency stability tests were conducted using the Spectran software, and observing the long term (about 3-4 hours) capture on the PC screen.
(click any pic to enlarge)
Here is the schematic of the VXO I made
and these are some mounting pics of my prototype:
VXO specification:
Frequency coverage: 9.985 – 10.234 kHz
Power output: about 500 uW (-3 dBm), taht's 450 mVpp onto a 50 Ohm load
Power supply: 6 V stabilyzed
Current consumption: 25-30 mA
Frequency stability: (after 10’ of warm up), no more than 5 Hz
for the first point: I setted my PC with Spectran software and QRSS mode, and put the signal form my FT817 receiver into the PC, in order to check it in a long time. After a warm up of about 10' I obtained a good frequency stability, no more than 10 Hz of variation as showed by the next figure:
about waveform, the TEK466 Scope and HP141 Spectrum Analyzer showed this result:
second harmonic level is just -12 dB under fundamental, and third harmonic level is about - 32 dB down. So not so good!
As soon as I'll make any modification I'll put the news here!
UPDATES: a new enclosure and a different Variable capacitor: herein the pic of the new box (clik to enlarge)
Next one will be a selective amplifier .
1 - With four Xtals I obtained a good swing (about 260 kHz), but the VXO is becoming like a good VFO, and frequency stability was not so good for QRS operation althoug it seemd a good VFO (measumement is in next figure):
1 - With just two Xtals I obtained a not so big swing (about 20 kHz), but the VXO frequency output was very good for QRS reception; in next figures you can see the measurement made at the upper and at lower freqeuncy range :
10.227 kHz measurement 10.227 kHz measurement 10.240 kHz measurement 10.220 kHz measurement
Spectran setup: QRSS Spectran setup: very high resolution Spectran setup: QRSS Spectran setup: QRSS
You can see that in the second screen the setup is for 1 Hz resolution and that in the measurement time (about 20') there was no more than about
4 -5 Hz drift; the last screen is the I0CG DDS VFO measure and the frequency is really rocky!
(warm up time was about 2 hours for VXO and about 30' for DDS VFO)
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Next screen are a couple o measures I made using Sptrum Lab by DL4YHF, that my friend Paolo I1DFS told me to try... very useful too:
CONCLUSION:
for QRSS and VLF RX the better is a XTAL with a very short swing (maybe the I5TGC solutions with two different XTALs mixed and shifting just few kHz, or a DDS VFO; or at least a VXO with a large number of different XTALS.... (I've not so many crystals!..)