Going back a few years, to at least the early 1970s, using separate transmitters and receivers was a fairly common practice among radio amateurs. By building "separates" you are duplicating some circuitry compared to a transceiver but it does give great flexibility. This receiver was built to replace a Drake R4A which had been used in conjunction with a simple crystal controlled transmitter. This receiver outperforms the old Drake by a significant margin and yet is fairly straightforward to build using off the shelf parts, modules and case. The details presented here are intended to assist others with previous building experience to construct a similar receiver, rather than being a step by step guide.
An IF frequency of 8.215 MHz was chosen based on having a pair of suitable filters and also a selection of crystals for that frequency. 8.215 MHz will mix with the minimum frequency of the synthesiser (10 MHz) to cover from 1.8 MHz upwards. "High side" oscillator injection is used on all bands. Although I have based my band pass filters on the 10 HF amateur bands, reception is possible anywhere on HF with suitable band pass filters, the exception being a small block around the IF frequency.
The performance is more than adequate with a two tone dynamic range of 94 dB at 2 KHz signal spacing, and an IF rejection of at least 80 dB down (87 dB IF rejection measured on 7 MHz, possibly the worst band). The image rejection is poorest on 28 MHz at 70 dB improving to better than 100 dB on 1.8 MHz. Only the top few amateur transceivers can better the dynamic range performance of this design. These figures could be improved at the expense of more complex band pass filtering and by using a stronger mixer with a higher level of oscillator injection. The minimum discernable signal (MDS) is -127 dBm (0.1 uV) on all bands without the pre-amp and a few dB better with the pre-amp.
At the heart of the receiver are two modules that make assembly fairly straightforward. The IF and AGC board is the "Hybrid Cascode IF Amplifier" by Wes Hayward (W7ZOI) and Jeff Damm (WA7MLH) from the December 2007 edition of QST. The second module is a synthesiser by Cumbria Designs used with their "mini counter" frequency display. The synthesiser generates an output of 10 MHz to 250 MHz at a level of +10 dBm, although the maximum frequency is set in the synthesizer menu at 38.215 MHz (corresponding to 30 MHz in this receiver). There are alternative synthesisers from other suppliers which cater for SSB/CW offset, have RS232 capabilities and that can also switch band pass filters, however I opted to use the Cumbria unit and also their counter unit, the latter can use up to six user chosen offsets to correctly display LSB, USB, CW, AM and if needed reverse CW. I manually switch the band pass filters after setting the frequency with the synthesiser. The audio amplifier is a Velleman K4001 kit.
I only etched two boards, the IF filter board and the AF pre-amp. The other boards were made using a series of "islands" or "pads" created with a 7mm diamond tipped tubular "drill" of the type used to drill ceramic tiles (mount your drill in a stand to avoid it wandering all over the board).
The case used is a "Unicase" by Metcase (www.metcase.com), it measures 260 x 90 x 250mm (W, H, Depth). These cases are stocked in the UK by RS Components. An aluminium sheet is fitted in the centre of the case, this is not supplied with the case. The knobs, including a 31.8mm diameter tuning knob, are from eBay suppliers. The "signal" meter is from Maplin, with illumination by a white LED.
As a minimum the BFO needs to be enclosed in a screened box, this can be made from scraps of print board. The IF and filter boards are very sensitive to stray signals and could benefit from being enclosed too. Some of the other synthesisers have a tin plated screening can as an option, adding screening to the synthesiser is probably a good idea.
The front panel was produced in Corel Draw and printed on photo card, which in turn was glued to the aluminium panel with photo mount spray adhesive. A clear plastic self adhesive "book protector" film was stuck over the card to protect it.
Some test equipment is essential to the building of this receiver, the following will be needed:
An L/C meter to measure capacitors and inductors (an LC200A from eBay is adequate and costs around £25), Signal generator, RF millivolt meter or spectrum analyser, low cost multi-meter and a frequency counter. A vector network analyser (VNA) could be used to align/test the band pass filters, not having one I used a signal generator and spectrum analyser.
For a view of the underside of the receiver click here, for an upperside view click here.
For an overall block diagram, click here (part 1) and here (part 2).
Band Pass Filters
There are 10 band pass filter boards selected by a 1 pole 12 way front panel switch, these are relay switched but diodes could be used instead. Each board is constructed on single sided PCB (avoids possible shorts to adjacent board), and individually tested/aligned before being added to the main board. The filter design was done using the freeware Windows program "Elsie", using a topology of "Mesh Capacitor-coupled band pass" and "Chebyshev" Family of 3rd order (uses 3 inductors and 5 capacitors per filter, actually produces "5th order filter"). The design was altered to use the nearest 5% capacitor value with inductors wound for the required inductance using a low cost L/C meter. For those not familiar with this program, beware of the default Q settings of capacitors and inductors being excessive (a value of 200 ~ 500 is more realistic) and also set the transmission to "Absolute", both these settings are found under the "Analysis" tab.
Capacitor and inductor values are critical, being off by a few percent can make a huge difference to performance. Unless you can source close tolerance capacitors, you will generally have to make up the required values by connecting 2 or 3 in parallel and measuring the value with an LC meter. The toroids should be wound for an extra turn and then fine adjusted by removing a turn or two and/or moving the wires on the core until you obtain the required value, estimating the value based on the number of turns will not produce repeatable results. For the higher bands, trimmers were used for the series capacitors (7 MHz and above). T50-2 cores were used from 1.8 to 10 MHz with T50-6 on the higher bands.
The frequency response graphs produced by the Elsie program were pretty close to my measured results. See the attached spreadsheet in pdf form for my calculated values, however, the Elsie program is so easy to use that you can produce your own values easily enough. Do not try to cover too wide a bandwidth when designing the filters, as the image and IF rejection levels are better for narrower bandwidth filters. However, if you attempt to make too narrow a filter, you will find it gives excessive loss. Typical measured loss figures for these filters vary from less than 1 dB to 2.5 dB. There is a series tuned circuit across the output of the band pass filter rail to ground which notches out the IF frequency, I used a 12 uH coil wound on a T37-6 toroid with a 65 pF trimmer capacitor.
Each band pass filter is "tack" soldered to a mother board, with the relay switched input/output soldered to a common bus bar.
Other bands could easily be accommodated as the filters are switched independently of the synthesiser, the only limitation being that you cannot receive near the IF frequency. Using this arrangement of filters, synthesiser and separate counter will allow operation to 50 MHz with appropriate filters. The synthesiser itself is capable of operation to more than 200 MHz.
Pre-amp and Attenuator board
These are fairly straightforward and conventional, they are both relay switched. There is 10 dB of attenuation and 12 dB of pre-amp gain available, selected by a centre off toggle switch. The pre-amp uses a 2N5109 bipolar transistor with heatsink. Click here for the circuit.
Mixer/post mixer amplifier
Again this is conventional, a TUF-3 double balanced diode ring mixer is followed by a 2N3866 post mixer amplifier (heatsink needed). There is a 3 dB attenuator on the synthesiser output to drop the +10 dBm level to feed the mixer and to provide a good 50 Ohm termination to the mixer. The post mixer amplifier design has been optimised for a good 50 Ohm input impedance, several similar circuits have a poor input SWR. The 0.02uH coil (L1) makes little difference and was a circuit modelling tweak. Click here for the circuit.
IF filter board
I was able to source a pair of Yaesu filters at a reasonable price, these are nominally centered on 8.215 MHz and are (hopefully) of 500 Ohm impedance, however any filters in the 5 to 12 MHz range could be used, beware that 8.2 MHz is the lowest you could use with a 10 MHz synthesiser to cover the 160 metre band. The filters are switched using two dual pole change over relays for each filter, there is a slightly increased loss when using the CW filter, I didn't add additional amplification to overcome that loss as the receiver is sensitive enough without adding another stage. Room was left on the board for the future addition of an AM filter.
IF amplifier and AGC board
This used a ready etched board offered by KA7EXM, However, as at April 2013 these boards are no longer offered. The original article (for those who are not ARRL members) can be downloaded from the KA7EXM web site (http://ka7exm.net/hycas/index.htm). The board was designed to be used with 50 Ohm filters, my IF filters are 500 Ohm so the input matching circuit wasn't used and a 560 Ohm input terminating resistor substituted. An S-meter buffer was added to the AGC output terminal, this is a single transistor and feeds a simple "signal" meter. The meter was adjusted for 50 uV at half scale, unfortunately at low signal levels the meter doesn't indicate but that isn't important to me. The mute input is used to turn off the receiver when transmitting, it was not found necessary to also mute the AF pre-amp. There are no clicks or thumps when muting the receiver via this input.
Product detector and BFO
This is another conventional board, I cheated and used one crystal with a "VXO" style of circuit to pull the crystal to the appropriate offsets for CW, LSB and USB. The BFO is followed by a low pass filter as there were significant harmonics into the high VHF range when checked on a spectrum analyser. An output for an AM detector is taken from the IF side of the SBL-1 mixer. The BFO transistor is turned off when AM is selected. The 1K output level pot is to match audio levels between AM and SSB/CW, when I add the AM filter and detector. It is essential the BFO is shielded to avoid oscillator leakage into the IF amplifier. Click here for the circuit.
Another conventional circuit, this uses an NE5532 dual op amp. There is no particular attempt to filter the audio, although the higher frequencies are attenuated to some extent, for a graph of the overall response, courtesy of the circuit modelling software "Tina", click here. While not shown in the image of the upper side of the receiver, a low level audio input is catered for in order to provide a side-tone from a companion transmitter. The circuit diagram is linked here.
Audio output amp
A Velleman K4001 kit amplifier was used, the output coupling capacitor was replaced by one of 100 uF to reduce the bass content of the audio and a precautionary 0.01 uF disc ceramic capacitor added across the input terminal to reduce the risk of RF pickup. The amplifier is specified as 0.05% distortion and a maximum output of 3.5 Watts RMS into 4 Ohms.
Synthesiser and counter
These are standard items from Cumbria Designs. The advantage of using a separate counter module is that it allows me to use any offset I program into the counter, the downside is the extra cost and the tendency of the display to flick between the 10 Hz digits. There are other synthesisers available which will do the job, some offer RS232 control and have band switching outputs too. A Bourns 64 pulse per revolution optical shaft encoder is used in place of the supplied mechanical encoder. I do not use a "push" switched encoder, the front panel "F/S" button is wired in place of the encoder switch.
Once left on a frequency for around one minute, the frequency is internally stored in the synthesiser -which allows the receiver to be powered up on the last used frequency.
Each band pass filter board was tested/aligned before adding them to the mother board. Those with trimmer capacitors were adjusted for maximum signal towards the higher end of each band, there was no need to stagger tune them. A final adjustment was done with the boards soldered in place. The IF notch filter was adjusted for minimum signal on 8.215 MHz with the 7 MHz band pass filter selected.
The BFO was initially set to the predicted offset frequencies followed by a final adjustment with the screening cover in place using a smart phone running a free audio spectrum analyser program (AKLite, an iPhone App) to measure an offset from a signal generator, a 1 KHz audio tone in the case of LSB/USB and 750 Hz for CW.
The counter can be adjusted using a known accurate signal, the synthesiser need not be accurately set.
The overall gain and noise level from this receiver gives very pleasant audio, the gain of the AF pre-amp produces just the right amount of audio. The AGC works very smoothly even if the S-meter lacks the ability to measure low level signals. There are one or two minor birdies, but none that cause any issues. Adding screening to the IF and filter boards (and possibly the synthesiser and counter) would have been a good idea.
I haven't added DC wetting to the relays, so far I do not have problems with intermittent contacts, but others have reported issues when using relays without a few milliamps of DC current passing through the contacts.
Thanks to Dave, GM4EVS, for the neat diagrams and circuit analysis and to W7ZOI/WA7MLH, for the IF amplifier design.
Please do not build this design without checking the values for yourself, in particular the band pass filter components. The information is presented here to assist experienced constructors who may wish to build something similar, it is not suitable for novice constructors.