C.42: LA7MI 4MHz variable
bandwidth xtal filter
based on negative impedance-amplifier.
The filter is based on a HP spectrum analyser (1980), but the original circuit diagram was almost impossible to understand and Stein (LA7MI)
had to go his own way trying to use the idea building a negative resistance amplifier.. The original filter operates on 21.4MHz with variable
bandwidth between 1kHz and 5kHz. LA7MI's circuit uses 4MHz xtals and may be tuned in set to bandwidthes from 0.3-3kHz. The
switch uses BA682 switching diodes. The xtals were from a bunch of 20 where he measured them as accurate as possible and used 4 which
had maximum 10Hz difference. The trimpot is adjusted such that the gain is equal for all bandwidthes, and the phasing potmeter is adjusted
using sweep equipment, other efforts may seem impossible to align the filter and may only cause an endless effort with little success.
The stop-band selectivity is only 25-30dB for one section, so you need 4 similar stages in a decent spectrum analyser. For higher bandwidthes
he uses LC-filters.
The unit is built on a 7x25cm pcb in SMD technique. MMBT3904=2N3904=BC237B, MMBT3906=2N3906=BC557B, SST310=J310.
I have no more component details, it is all based on some telephone conversations and a sketch he sent me via snailmail after he mentioned
it yesterday and I said I was curious about the circuit diagram for the negative output impedance amplifier. The amplifier is adjusted to
reduce the loss resistance of the xtal, so if it had -47ohm output resistance and you put a 47 ohm resistor in series with the output from
the amplifier the total impedance is zero.
Notes which are not signed otherwise comes from
He has a soldering iron, but no email, and is reached by telephone nr +47-55-902392.
He will be very pleased to receive comments, the postal address is: LA7MI Stein Torp, Tollbodalmenning 34, N-5005 Bergen, Norway