This will be a superhet with 4433kHz IF (5 xtals, 250Hz Cohn)and run low side LO with a Hartley VFO.
Power out will be in the 10 watt range.
LM386 audio chain for the RX path.
TX carrier osc at 4433.50 kHz
Left to right:
RX IF downconvert mixer (TUF-1) in to common base NPN audio amp,
then common emitter audio gain in to FET switch for QSK muting.
RX Adjustable BFO -- An NPN Colpitts XTAL osc with series air variable in series with the xtal.
Another view of the RX BFO. Note the DC feed through cap for the power supply.
A 3 terminal voltage regulator is mounted outside the box.
Serious grounding for the lid ensures that BFO leakage does not get in to the RX IF amp chain.
A new superhet chassis. This developed in to the following 75m CW/SSB transciever over the 2005 winter.
The LSB/USB BFOs and CW carrrier osc.
These still need to be fully enclosed.
I left more room than I needed to just so that I would not get too cramped.
This is the shaped keying circuitry.
The +12 keyed line is a single 2N3906 based integrator inspired by W7EL.
The +12 transmit is simply a 2N3906 pull up for base current to the TX amp stages.
The +12 receive line is a separate inverted version of the +12T line that facilitates muting the MIC input
during receive.
This rig has high side LO injection (13.5 to 14 MHz). The IF is at 8 MHz with a 10 xtal filter
in each path using .1dB Chebycheff response for approx 3kHz bandwidth at the -6dB down points.
The separate filter in each path approach allowed me to keep some flexibility to experiment with
the receive path without altering the transmit path response, or vice versa.
In hind sight, I found the separate filters to be a nuisance in that the skirts are not as easy to match as
I had expected/assumed.
I used xtals with unloaded Qs of greater than 160k each for the receive path, and xtals of greater
than 142k in the transmit path.
These two filters were built out of a purchased set of 100 Fox brand microprocessor xtals.
Each xtal was measured for unloaded Q, series R, motional inductance, motional capacitance and
parasitic parallel capacitance.
The filters were designed using the software written by Wes Hayward, W7ZOI, that comes with the ARRL
published text "Experimental Methods in Radio Frequency Design".
Each filter mesh (xtal and associated caps) were custom tuned to account for slight descrepancies in xtal
resonance so that each mesh became tuned to the same frequency.
This detail allowed the filter to be designed and built to have the expected passband ripple and insertion loss.
The latter attention to detail allowed the filters to come in at around 2.5dB of measured passband insertion loss,
compared to about 2.2 to 2.3 dB of calculated insertion loss.
My home brew measurement gear is hard pressed to offer better than .5 dB of resolution accuracy,
which is actually quite alright for home made test gear.
A view of the Hartley VFO mounted on the side of the ARC-5 tuning air variable.
A view of the VFO premix circuits to get a 6 MHz VFO up to 12 MHz with a
diode doubler and band pass fiter and buffer.
The graphics from GPLA, the software that comes with the ARRL text
"Experimental Methods in Radio Frequency Design".
A shot of one of my early filter design verifications using 10 MHz xtals.
This experiment was based on mica trimmer caps in order to allow accurate capacitor values
to be acheived in order to study the real world performance compared to simulations.
The results were very satisfying in that there was very tight correlation between simulation data
and measured results.
The MIC amp using a cascade of LM741 opamps and the DSB balanced modulator board.
All of the transmit and receive mixers are double balanced diode rings.
PIN diode switching was used to share the BFO and VFO signals to the mixers.
The poor isolation of high voltage rectifiers as PIN diodes is not a problem when
switching oscillator outputs.
Their poor isolation is a huge problem as SPDT switching of 2 port networks like filters.
The last part of the TX amp chain.
Broadband design was used throughout to allow easy band changing in the future.
Replacing 3 band pass filters and a low pass filter are the only changes necessary to change bands.
If all goes well, I will be posting schematics in the near future.
The outboard final amp is a broadband single ended IRF510 intended to run on a 35v power supply.
With Vdd=18v, this will put out about 19 watts, which does well for casual close in contacts.
(500 mile radius)
75m was chosen as a good winter band for late night on air signal checks.
The rest of the HF bands have marginal use late at night with a low elevation antenna and low power.
May 2007 : The new MT shack.
