An Improved Superhet for WRMI reception
This, hopefully, will be a "new and improved" version of the simple superhet for reception of WRMI that was previously shown here.
Improvements will include wider range frequency coverage, better image rejection, and improved selectivity, achieved principally by changing from a single down conversion to an intermediate freque3ncy of 455 KHz, to a dual conversion up converting to 10.7 MHz as the first intermediate frequency.
With the use of a simple low pass filter as the front end, reception from the broadcast band to seven MHz or so should be readily accomplished. Images will be in the range above 22 Mhz so unlikely to present any issues, and a crystal filter at 10.7 Mhz will establish the initial selectivity ahead of the second IF.
Construction has begun. The Front end filter, mixer, and second conversion local oscillator are complete.
A TDA1072 again is used as the main device of the receiver. Unlike version 1, if successful, this second revision will use the onboard TDA`1072 RF amplifier as the first intermediate frequency amplifier, and the TDA1072 balanced mixer as the second mixer. The first mixer continues to be a mini circuits ADE-1+.
The crystal oscillator gave me fits. The TDA1072 data sheet says next to nothing about the onboard oscillator. No schematic of the chip is given, and I have been unable to locate one. Ultimately, I got the circuit shown in the data sheet to work, and work well. Prior to that I spent considerable time trying to get a crystal to emulate the tank circuit shown in the data sheet for a variable oscillator. This was utterly unsuccessful and I believe the data sheet crystal oscillator is likely the only circuit that will work. The second local oscillator frequency is 10.7 MHz.
My notebook pages for the work thus far are attached. I usually do not draw schematics as I tend to think of things in small blocks of circuits.
The low pass filter is a three pole circuit, with a measured -3 dB point of 7 Mhz. 50 ohm -2.5 dB attenuator pads at the mixer input and output establish a broadband 50 ohm termination for the mixer. A binocular core transformer will be used to step up the 50 ohm output impedance of the mixer to the higher impedance typical of narrow band monolithic crystal filters. Sweeping the transformer shows a relatively flat response across the 10 MHz band, with about an 8 dB rolloff beginning in the image range, so it will additionally provide some contribution to image rejection.
The low pass filter shows a bit of a bump around 23 -24 Mhz before recovering and beginning to attenuate again. I suspect this is related to the self resonant frequency of the 1.5 uH inductors in the filter.
Most of the parts are 0805 smd, but the oscillator uses a mix of leaded parts, and 1206, 0805, and 0603 smd parts. Some smd parts are located under the through hole parts and are not visible.
73,
Win W5JAG
oops: I just noticed I put in my notebook the attenuator is a T arrangement. Obviously, it is a Pi arrangement with the 15 ohm part the series element, and the 300 ohm resistors the shunt.
Also, the oscillator page had a serious error. The attached version is now the correct version.
Over the weekend, I switched out the toroid inductors in the front end for much smaller SMD inductors, a very worthwhile change, I think.
I also installed a tentative AGC controlled PIN diode attenuator between the first mixer output and the input of the MAR-6SM IF amplifier, and am in the testing / adjustment phase of that.
I am starting to work on some additional documentation. Here is the graphic of the audio filter response at the output of the TDA1072 with the presently installed components. It matches up nicely with the 6 KHz Murata filter. I had Google AI generate the graph, so take it with a grain of salt, but it does correspond with what I hear.
73,
Win W5JAG