CG-1 A calibrated Comb Generator by SV1OBT
Today I am going to present a comb generator that I designed and tested a while ago. For those who don’t know what this is, basically a comb generator circuit produces multiple harmonics out of a given reference input signal. For example, if the input signal is 1MHz, the comb generator will produce harmonic signals at 1, 2, 3, 4MHz, etc.. In my circuit the reference signal is included and is set to 100KHz, so we get a ‘comb’ looking spectrum with ‘teeth’ spaced every 100KHz.
For more information have a look here
Many times amateur radio enthusiast would like to have the means to produce a reference signal of known power to use it in their experiments.
Access to a commercial RF signal generator is not always an option for most hobbyists.
With the help of a comb generator, with calibrated output, one can use the harmonic signals as a reference signals of known frequency and power.
For example to calibrate the S-meter of a receiver or to check the response of a filter or maybe test a RF amplifier stage, or align a transceiver… ….there are many applications for this little tool.
If someone needs a particular pure sinusoid signal, a bandpass filter can be used to select the signal of interest. (Provided that the insertion loss of the filter is known, so that the selected signal’s power is known after the filter).
I designed the generator to give harmonics every 100KHz, and to produce harmonics of useful power from 1 MHz up to 1.42 GHz. Once I was satisfied with the spectral content, I calibrated 16 harmonics which were carefully chosen so that they represent signals in many ham radio bands. The idea was to have a useful signal at every HF band, 2m, 70cm, 23cm, and finally a signal at 1.42GHz for ‘Hydrogen Line’ detection experiments :-). The accuracy of my calibration is +-1dB referenced to my calibrated spectrum analyzer with total amplitude accuracy of <1.0dB (95% confidence level). This is not a certified calibration by professional laboratory, but it serves the hobbyist rf experimenter just fine for most needs. Giving the generator to a friend to double check the signal levels in his lab revealed that my measurements were consistent and within specification, so I am happy with that.
The comb generator produces thousands of harmonics between the selected (calibrated) frequencies. The signals falling close to the calibrated frequencies are most of the times quite similar in terms of amplitude level.
Here is a table with the selected frequencies for calibration, and corresponding power level for each one of them.
Frequency (MHz), Power (dBm) Useful for testing Band
1.900 -46.6 160m
3.600 -42.1 80m
7.100 -38.9 40m
10.001 -38.2 30m
14.101 -38.1 20m
18.001 -38.1 17m
21.201 -38.5 15m
24.801 -38.9 12m
28.401 -39.7 10m
51.003 -46.7 6m
90.005 -52.3 FM broadcast
125.906 -59.0 AIR
144.907 -66.6 2m
435.022 -83.9 70cm
1295.967 -97.2 23cm
1420.373 -98.0 Hydrogen line
Example application
If lower power levels are needed, one can connect an external attenuator to further attenuate the signals to any level. To test the s-meter of my FT817, I used a 30dB attenuator and got an S8. The signal’s power after the attenuator was -66dBm -30dB = -96dBm.
For a s-meter display vs received power (dBm) have a look at these tables
The specifications for the finished generator are shown below
Specifications
Battery operated unit. (9v battery on a battery holder for firm mounting)
Small, aluminum die cast box with reliable switch and quality SMA connector mounted on the box.
RF shielded design to keep unwanted noise out and being able to produce very low power harmonic signals.
Frequency range of harmonic content: Every 100KHz from 100KHz up to 1.42GHz (to cover all HF, VHF, UHF, SHF bands)
Power level range : Between -38dBm and -100dBm usually. (differs a little bit between each serialized unit)
Power level accuracy: +-1dB (each generator has its own serialized calibration sticker with its measurements). Calibration is performed with the use of a calibrated 2cm rigit cable.
Frequency stability: 20ppm crystal oscillator reference at 20MHz.
Output signals are immune to supply voltage deviation due to battery draining.
Being able to provide >100 hours of service with an alkaline 9v battery
if you are interested in this project, you may contact me at info@jel.gr
Calibrated frequencies are printed on the sticker on the top lid.
