Dear Jim,
since about 2 years I use a direct conversion receiver for 136 kHz. The
schematic may be downloaded from my web page http://www.qsl.net/ik2pii . So I
expose my humble opinion in the matter.
1- Front-end. Here, north Italy, we don't have strong signals near 136 band,
DCF39 is not so strong as in Germany. Now in my receiver I use 4 tuned
circuits, with strong high frequency attenuation (46 dB at 200 kHz and 80 dB
from 500 kHz to up). This configuration attenuate MW broadcasting signals. This
front-end also attenuate the image frequency.
2- Mixer. Thank's to the well designed front-end I can use an NE602 IC mixer,
not a champion of IP3 but with some gain. I don't have intermoduation problem.
3- Local Oscillator. As on schematics on my web page. The NE602 work well with
square wave local oscillator.
4- AF coupling and amplifier. In my ultimate RX version I use only the first
OP-AMP, a low noise NE5534. AC coupling, with high pass corner at about 300
Hz, is very useful to attenuate 50 Hz signal and the second and third armonics.
The output of the op-amp go to the PC audio board through a 600 ohm 1:1
transformer coming from an old modem. This help to reduce 50 Hz coupling
between RX and PC. Note that this transformers are linear from 300 Hz to 300
kHz!
5- Tests. I also have a selective level meter Wandel&Golterman SPM-15. Direct
conversion RX and W&G, with a 200 kHz low-pass filter in the input, are
comparable. The narrow IF filters (up to 25 Hz bandwith!) of the W&W are not
useful if you don't have a strong signal output in the audio band, exceding the
dinamic range of the audio board.
I also noted that the AGC circuit of HAM equipements is prejudicial in receiving
weak signals. Statics and signals that activate AGC reduces the sensitivity of
the receiver. So I prefer DC receiver or selective level meter.
On 19-Feb-02 James Moritz wrote:
Re: Andy's comment:
...how about
the next version, a direct conversion receiver generating quadrature signals
at 137kHz then having masses of low frequency gain before the X/Y plot ?
A low noise, high gain version of the circuit would certainly be possible,
but there are a number of problems to be overcome here:
The signal spectrum at the mixer output extends to DC, so any DC offsets
will be amplified as well as the wanted signals. This would saturate the
output unless offsets could be kept down to a few uV. Even then, the
remaining offset would effectively be an unwanted 0Hz "carrier" in the
demodulated output spectrum. A related problem is that the noise figure of
amplifers, etc. increases drastically at very low frequencies due to
flicker noise. Both these problems could be avoided by having AC coupling,
but this would put a hole in the received signal spectrum around the LO
frequency.
Also, if we low pass filter the I and Q channels in order to define the
bandwidth, any mismatches between the filters in the two channels would
result in errors in the phase quadrature between the 2 channels, which
would be a problem if good image rejection between the sidebands were
required in subsequent demodulation of the signal.
I think that if mismatches are stable and measurable the software can take in
account the mismatch and correct for it.
I think it would probably be possible to achieve a useable result for a
dedicated 136kHz RX - a fairly simple bandpass filter before the mixers
could restrict the bandwidth to a few kHz, and some pre-mixer gain could be
applied, reducing the post mixer gain required and reducing the effects of
offsets.
Cheers, Jim Moritz
73 de M0BMU
I think that you had a very good idea. DC receiver with I&Q channels driven
into an audio board may be the ultimate LF receiver. You vcan also see on the
web site of SM5BSZ (search for "Linux pc radio") some useful pages about his I&Q
DC receiver for 144 MHz and the DBM mixer to audio amplifier problem.
73 and good thought.
Claudio, ik2pii
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Claudio Pozzi http://www.qsl.net/ik2pii happy Linux user
E-Mail: Claudio <[email protected]> <[email protected]>
Date: 19-Feb-02 Time: 19:07:15
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