I am now putting the finishing stages to a custom LF receiver, designed to
be a useful accurate measurement tool as well as for operating. The design
is optimised for narrow band and ultra narrow-band working, but with an
audio bandwidth suitable still for CW. An A/D converter is included as an
integral part, with formatted data sent to a PC via the serial port making
soundcards, with their poor frequency stability and difficulties in driver
software, unnecessary. ALL frequencies and A/D timing are derived from one
master oscillator which may be an onboard TCXO (< 1ppm) or a highly stable
external 5 / 10MHz reference.
The basic receiver line up is as follows :
A 7th order LP filter cutting off at 150kHz to give greater then 100dB
rejection of the image frequency in the MF broadcast band.
LPF cutoff frequency could be higher, but we have very high power LF
broadcasting in Europe and I didn't want to tempt the
possibility of overloading if close to a transmitter.
MAR-3 modamp RF amplifier driven at constant current supply for temperature
stability and interference rejection at low frequencies
SRA-8 Double balanced mixer, upconverting to 455kHz IF
Low phase noise LO derived from a DDS module over-clocked at 10MHz, with
extra filtering to eliminate spurii
Automatic External / Internal reference switching, with frequency doubling
of a 5MHz signal if needed
300Hz bandwidth mechanical filter at 455kHz
IF gain provided by an Analog Devices AD603 prescision voltage controlled
gain stage, from a manual gain control pot.
This chip is laser trimmed to provide 10 - 50dB gain with 0-1V and
enables relative gain to be determined very
accurately, to a fraction of a dB by just measuring the applied voltage
with a DVM. AGC is not an option in a measurement Rx !
A BFO at 456kHz derived from the 10MHz reference mixes the IF to 1kHz centre
frequency.
Audio Bandpass filter at 1kHz reduces the bandwidth to 100Hz
12 Bit A/D converter sampling the 1kHz audio at 4kHz, generating 14 bit
Complex I / Q pairs at 1kHz sampling rate, centred on 1kHz. ie a 1kHz tone
appears in the data as if it were at zero frequency with up to +/- 50Hz
sidebands
Sensitivity of the receiver is sufficient to be used directly with a 0.8m
loop (at least for urban noise levels) without having too much gain that the
IF has to be wound back too far when connected to a full size antenna.
The receiver hardware is working perfectly, but at the moment tuning is only
under computer control of the DDS and I still need to add a manual tuning
mode. This will be 10 turn potentiometer which, via an 8 bit A/D, can set 0
- 255 frequency increments from a base value. Channel (increment) readout
can use the same DVM module already in use for gain setting, by measuring
the voltage directly. Both Base frequency and Increment will be
programmable from a PC and stored in non-volatile memory in the PIC
controlling the DDS, ready for instant turn on. Doing it this way means the
simple addition of the A/D chip to my existing DDS module with new PIC
software. By setting an increment of, say, 10Hz, the whole 137kHz
allocation can be covered without reprogramming, as well as most of 73kHz
for aural CW listening. Alternatively the DDS LO could be replaced with
the other designs around that make use of rotary encoders and LCD displays.
When finished a write up will follow which will hopefully be published in
one of the magazines, QEX or RadCom - haven't decided who to send it to yet
!
This is not a cheap low cost receiver just for routine operating, but has
been designed to be almost a laboratory grade piece of equipment useful for
precise frequency and amplitude measurements.
Andy G4JNT
-----Original Message-----
From: Andre Kesteloot [mailto:[email protected]]
Sent: 2001-07-31 12:58
To: rsgb_lf_group
Subject: LF: LF Receivers
We have posted on our website at
<http://www.amrad.org/projects/lf/>.
some details of the new AMRAD receiving antenna that will
appear in QST
for September.
In addition you can now also find there the complete
instruction manual
for the Cubic 3030 receiver, in PDF format. (The Cubic 3030
is a US Mil
Spec receiver that covers from 5 kHz through 30 MHz. Both Sandy WB5MMB
and I own one).
73
André N4ICK.
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