Page last updated May 20 2018
The images below show the output sine wave and harmonic content from the VFO before and after filtering with the lowpass filter with an FC of around 6.5 MHz. The filter was modelled with 'elsie' from Tonne Software.
A basic low-pass filter was modeled to add to the output of the VFO to 'clean up' the output
The assembled Lowpass Filter
The filter has provision for a resistive attenuator at the output to reduce the VFO level after filtering to around +7dBm to the present to the local oscillator input of the SBL-1 balanced mixer. Values were selected to provide 4dB of attenuation. I used NPO ceramic capacitors which were on hand.
Marker 1 = 5.0 MHz - RL = 18.5 dB
Marker 2 = 5.5 MHz - RL = 28.5 dB
Marker 3 = 10.5 MHz - RL = 0.15 dB
Marker 4 = 21.0 MHz - RL = 0.002 dB
Marker 5 = 6.5 MHz - RL = 6.3dB
The Progressive Communications Receiver from November 1981 QST
There is significant distortion from the output of dual gate FET VFO buffer, I temporarily reduced the coupling capacitor from 10pF to 6P8 with little improvement. Also adjusting the gate 2 voltage had little effect except for reducing the output level so it was decided to add some filtering to the output as there is adequate output level. See below.
The first steps in the project. The I.F. amplifier and VFO. Some photos below.
More progress(ive) updates to come!
A bit more progress today due to having a pre-formed chassis in the 'junk box' that I had recently earmarked for this project so it has now been pressed into service. I acquired this chassis in 1998 for another project that seemed important at the time but did not eventuate. I also had an unused Eddystone 898 slide-rule dial that is suited to this project too. Below are some photos of today's (Saturday afternoon's) endeavours. Fortunately for me, I had the original drilling template for the dial. Nibbling out the rectangular hole in the front panel was a time consuming process that I was not looking forward to but after an hour or so it was done! Just a bit of filing afterwards to tidy up the cut-out.
Saturday, 14th April 2018
The next step for this project is to settle on the rest of the receiver's layout before drilling too many holes in the chassis that may not be needed or drilled in the incorrect place! For the front panel, I need to allow for an S-meter, band change switch, AF and RF gain controls, preselector peaking, as well as a headphone socket, RIT etc etc..... Anything else? A notch filter would be nice too!
Today I drilled some PCB mounting holes in the chassis and also etched and drilled some more printed boards for this project. The 80 metre preselector/mixer and the convertor mixer board as well as the 40, 20, 15, 10 metre converter boards. Some photos of the boards prior to drilling are displayed below.
Sunday, 15th April 2018
Today I finished loading the components onto the detector/audio PCB. My version of these stages are different from the original article in that I've used an LM386 as the audio power amplifier. I have not tested this variation prior to this PCB build but I will test, verify and post my results here. My concern is whether I will have enough audio level to drive the LM386 effectively. The LM386 is not the best of choices as far as distortion and output power but it does do a reasonable job and I have a number of them in the parts bin. I've also had a bunch of the Mini-Circuits SBL-1 mixers in my possession since the early 1980's and used very few of them until recently. My task is to load the components onto the 80 metre mixer PCB as well as the 2nd mixer PCB.
Over this weekend I also spent a couple or more hours on the chassis layout and where to locate some more of the printed boards as well as work out the 80 metre preselector tuning physical layout. I needed to keep the preselector capacitor as close as possible to the preselector PCB which is situated directly behind the VFO compartment but also needed the adjustment knob to be in a suitable position on the front panel. I had the two pulleys and belt already. They were left overs from another projects from years ago.
The two variable capacitors are identical with a maximum capacitance of 420pF. I would have preferred physically smaller ones but these are what I had in my parts collection. It's good to be able to put them back into use finally!
Friday, 20th April 2018
I've finished loading the components onto the 80 metre preselector/mixer board, the converter mixer board and the BFO board. I'd really like to use crystal oscillators instead of a DDS option for the converters but as yet I have not investigated the feasibility and cost of doing this.
A PCB artwork has been completed for a relay board which will take care of band-switching instead of using a multi-wafer rotary switch which I don't have. A small amount of DC current will be passed through each of the relay contacts whether open or closed. This to aid in contact longevity.
Getting closer now to be able to do some testing of the core 80 metre receiver. The RF and AF gain control pots were purchased from my local Jaycar Electronics store during the week so will fit them and begin wiring up the PCB interconnections at the weekend.
This afternoon the rear panel was fitted and some more holes drilled into the front panel to accommodate the various potentiometers and the 'S' meter. I'm going to use an old style 500uA FSD for the S-meter. I'll have to redo the existing 0-500 scale to make it look like a signal strength meter.
Wednesday, 25th April 2018 (Anzac Day)
Saturday, 28th April 2018
A power supply PCB artwork was designed last night. I allowed for three supply rails. +/- 12V and 9V. Actually there are two isolated +12V rails. I'll dedicate one to the detector and audio amplifier PCB to improve isolation and decoupling from the other receiver circuits. I'm not sure yet if I will need the -12V and +9V but who knows what additional features may get added in the future!
A bit more time spent on the front panel this afternoon. I fitted some of the controls and S-meter etc. to review the layout.
The 80 metre mixer stage was checked this evening with the spectrum analyser and tracking generator. Focusing mainly on the passband filtering stages ahead of the SBL-1 mixer. The preselector tuning capacitor adjusts the peak response between 3 and 4 MHz and that seems fine. However the skirts are not as steep as I expected and the attenuation either side of the peak not overly deep. The low pass section of the filter is functioning as expected. Insertion loss was measured as 6dB at 3MHz and 9dB at 4MHz. Allowing for some mismatch with my sampling technique let's say 3-6dB across the band. I will do so more tests with a 2nd board I have built and the output of the filter terminated not with the SBL-1 mixer but a 50 Ohm 10dB attenuator at the input port of the analyser.
I'm curious to know whether the same (or lesser) attenuation results can be achieved with a broadband filter design.
Power Supply PCB Overlay
Band Switching Relay PCB Overlay
A High Performance Communications Receiver
Amateur Radio Station VK2FC
Powered up for the first time as a 'complete' receiver. With just a couple of metres of hook-up wire as an antenna I could hear some weak SSB signals on 80 metres. A quick check with the signal generator revealed all is OK and it can 'hear' down to at least 0.1uV. Audio from from my modified audio board using an LM386 sounds clean and distortion free with adequate volume for the 4" speaker.
1st May 2018.
PCB artworks have now been done for the two boards below. I hope to etch, build and fit these at the weekend. The overlay on the left is a 6/12db 50 Ohm attenuator which can be switched in or out as required and provide either 0, 6, 12, 18 dB of signal attenuation. The overlay on the right is an S-meter driver circuit. A bit of 'googling' found the circuit on a W7ZOI web page. It is intended for use with the I.F. amplifier used here or the HYCAS I.F. amplifier circuit. My build of the HYCAS I.F. can be seen here.
Saturday, 5th May 2018
More front panel controls have been added as well as some grommets around the chassis to assist with cabling between the printed boards. The attenuator and S-meter printed boards were etched today and mounted in their respective locations. The attenuator is located on the rear panel adjacent to the antenna connector and the S-meter PCB has been mounted on the under side of chassis directly below the I.F. amplifier. This is a convenient location to pick up the AGC Voltage from the I.F. board and keep the interconnection wire as short as possible.
I've been giving more thought about where & how to place the four preselector boards, mixer board, band change relay boards and the four crystal oscillators. The only place for most of these to go is on the underside of the chassis. The underside is only 40mm deep. I already have the power supply board and meter driver board in position there so a total of 12 boards to be housed on the underside. It will be tight!
The PCB artwork for the crystal oscillators was completed early on Saturday. I was able to fit three oscillator circuits onto this size PCB so I will need two boards for the additional 5 bands. The board dimensions are 100mm x 60mm. A quick 'touch-up' with the pen in a couple of places and the board will be ready for etching.
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Not too much progress today due to other commitments although I did manage to etch two of the heterodyne oscillator PCB pictured above and make an aluminium plate/bracket to hold the 4" 4 Ohm speaker on the right rear side of the chassis. It sounds simple but took me exactly 1.5 hours to measure, manufacture and fit this plate and speaker to the chassis!
Sunday, 6th May 2018
Tuesday, 8th May 2018
The two crystal oscillator boards have been drilled and I've settled on the 5 bands I wish to add to the core 80 metre receiver. I will order the crystals today as it is a 5 to 6 week wait from Hy-Q Crystals in Melbourne. The bands are 40,20,15 and 10 metres, however 10 metres will be two 500KHz segments from 28 to 29 MHz. I'm uncertain as yet whether one preselector PCB will suffice or I'll need two to achieve the necessary bandwidth so will do some testing.
I had considered the idea of adding a 6 metre converter for one of the bands but I'll save that idea for another project.
5 crystals were ordered yesterday and now a 5 to 6 week wait until they arrive. I'll use this time to finish off the wiring of the core receiver as well as load the remaining boards with components. I also need to give consideration as to where they will be placed on the underside of the chassis which is only 40mm deep.
The image above is one of the filter/preselector boards that are required for each of the extra bands. I'll work my way through finishing each off and doing an alignment to see how the pass band looks. I plan to utilise the 40673 MOSFET RF amplifier for all bands except 40 metres which will have no gain.
As a bit of a diversion, I redesigned the VFO low pass filter today and was pleased to see significantly lower harmonic levels at the output as compared to the output from the previous filter as can be seen on the left. The 2nd harmonic is now almost 57dB below the 5 MHz VFO carrier as opposed to 38dB previously. I guess it's of no real benefit but I did it just to see what could be achieved. The new filter circuit is below.
Friday, 11th May 2018
Saturday 12th May
All four preselector boards have been assembled as per the photo below. The leftmost is the 10 metre board and the rightmost is the 40 metre board. A 40673 RF amplifier has been included for all bands except 40. The board on the lower right is one of the two 'band switches' that will select the preselector for the appropriate band. A similar board will connect the crystal oscillators to the 1st mixer board.
All four preselector boards have been aligned with a spectrum analyser and tracking generator and the plots can be seen below. The alignment was a bit difficult to get the desired response. It would have been very time consuming without the test gear mentioned above. None the less, it can be done by monitoring either AGC Voltage or recovered audio and tuning at the centre and upper and lower band edges.
A good starting point for the alignment is to adjust the coupling trimmer capacitor for minimum capacitance then peak the remaining trimmer caps for maximum gain at the upper band edge. Once done adjust the coupling capacitor for the desired bandwidth monitoring the lower band edge frequency. A further small adjustment to the bandpass tuning is necessary to get some 'balance'.
I'm curious to know why the 40 metre filter response's upper skirt is the shape it is. I think it may be due to the low pass filter fc being too high. The overall attenuation is also too high so further investigation is warranted.
I had concerns about the 40 metre preselector attenuation & response above so I used 'Elsie' from Tonne Software to model a bandpass filter for 6.8MHz to 7.3MHz which is going to be the tuning range for my receiver's 40 metre band. Referring to the images below.
1. Is the original first-built response
2. Is the response of the modified filter
3. Is a sweep of the bandpass section only of the modified filter
4. The yellow trace is the bandpass filter only and the green trace is both bandpass and lowpass
Notice there is less attenuation now and a flatter response from 6.8 to 7.3MHz so I'll go with this modification.
I started this project in 2016 by building the I.F. amplifier using a 9MHz SSB filter from an old commercial HF radio. No further progress was made until early April 2018 when it was decided to get a bit more serious about devoting some time to finishing this receiver project.
The circuit below will provide the switching to enable the antenna to be connected directly to the 80 metre receiver front end when the band switch is set to 80 and will connect the antenna to the filter switching relay boards and 1st mixer when any of the other bands are selected. I've provided for a small amount of D.C. to flow through the relay contacts when activated.
Monday 21st May 2018
I had a couple of hours to spare so made up a sub-chassis to mount the 40, 20, 15 & 10 metre filter boards on the underside of the main chassis. The idea behind the sub-chassis is to avoid all of the other M3 bolts & nuts already mounted through the main chassis. Six more PCB's to befitted on the underside. It will be tight!