27.125 MHz (third overtone) xtals are one of the old standard radio control frequencies so are easy to find (e-bay etc but remember to use the 'Tx' not the 'Rx' of the pair). However for some reason 27.145 MHz was not allocated to the AM CB service and this channel was skipped making 27.185 MHz channel 19. With 10 kHz spacing this should mean that MSF should appear 6 channels up on channel 20.
The old AM CB radios used 27.125 MHz as channel 14. The mixer takes the 60 kHz signal (0.06 MHz) passed on by the i/p stage and combines it with 27.125 MHz to convert the MSF signal to 27.125 + 0.06 = 27.185 MHz (or alternatively 27.125 – 0.06 = 27.065 MHz). If these are increased, to say 1nf, it might give better sensitivity for less selectivity which may make the filter less sensitive to temperature changes etc. The two 220pf capacitors around the tuned circuits will have quite a large reactance at 60 kHz ca. At a particular setting the preamp will burst into life – it's a nice moment! If you can, get hold of a capacitance box to switch through till you hit resonance. When I was at the NPL I spent hours trying to get apparent copies of this circuit to tune to 60 kHz. Note: I have found that the tolerance specification on inductors to be quite unreliable. If possible use fixed tuned caps to get the tuning within the range and then use the ferrite cores to tune for maximum signal. 4000 to 5000 pf to get the inductors to resonate at 60 kHz. It uses a FET and a couple of pot-core inductors (L) to form a narrow band pass filter. The input stage provides some gain and good filtering. I used an aluminium metal box for the converter and used coax inputs and outputs. The product goes to a simple output filter to feed the receiver. The 60 kHz signal from the antenna passes through a pre-amp and filter stage into the mixer where it is combined with the LO produced by the 602's internal oscillator (and an external xtal). The converter design is based around the NE602 double balanced mixer. with the tuneable VLF ferrite rod antenna described last time).
Vlf receiver sensitivity spesifications mod#
I am not sure if this was really necessary for MSF but the mod may come in handy when monitoring other VLF signals etc (e.g. This improved selectivity but at some cost to sensitivity. I changed the 10.7 MHz ceramic IF filter in the CB for a crystal filter. I have found it gives good stability and excellent sensitivity. I had an old Formac 88 from my radio repair days at GWM Radio and so decided to use this. In addition you can easily wire into the receivers AGC S-meter circuitry to use it as a detector for example. Old CB radios may not make a very compact set-up but they are easy to open-up and modify (no surface mount components) and they usually have plenty of space inside to add circuits (the converter for example). But why monopolise a great shortwave receiver, or the shack transceiver, to continuously pick up the 60 kHz signal so it can't be used to do anything else? Why not go on e-bay and buy an old AM CB radio for a few quid and build a simple converter and therefore make use of the transceivers double conversion superhet receiver, AGC circuits (S-meter) and other functions? However as most of us (radio amateurs) already have versatile SW radios of one sort or another I have describe here a very basic converter to pick up the MSF signal. DC and single conversion superhets receivers work really well and are worth exploring.
Since my year at the NPL I have built various types of receivers for reception of the VLF 60 kHz MSF signal including: Tuned Radio Frequency (TRF), Direct Conversion (DC), single and double conversion superhet receivers. Part 4: simple VLF 60 kHz MSF receiver converterīy Dr Jonathan Hare, The Creative Science Center, Sussex University. The Creative Science Centre - by Dr Jonathan P.
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