The 455 IF - Obsolete?
Absolutely not! Consider the environment we ham in. Foreign made transceivers run by their own internal computers, with a phase-locked loop (PLL) VFO, memories, super filters, DSP, VHF IF'S, and etc. It's difficult for the homebrewer to impress these guys and entice more fellows into the homebrewing family.
But wait for a moment! The 455 IF had a lot of features that can make a homebrew receiver sound better than anything on the market.
Remember the Q-Multiplier? This long lost feature had the ability to pull signals right out of the noise. In a magazine article in Ham Radio many years ago, the author stated that the absolute best method to improve the signal-to-noise ratio was with a Q-Multiplier.
The Drake 2B, manufactured in the 60's, had a Q-Multiplier option. The Drake receiver had a nickname, the poor man's Collins, because it almost performed as well as the top of the line Collins gear, especially with the Q-Multiplier. The thrill of tuning across the passband and having that weak DX signal jump right out of the speaker was spellbinding.
A Q-Multiplier raises the Q of a tuned circuit in the IF strip. This is difficult to do at high IF frequencies. Therefore, you only saw Q-Multipliers at IF's of 50, 85, and 455 kHz. The homebrew receiver also has the advantage of not having a PLL VFO, which means that a homebrewer does not have VFO phase noise so bad that the dynamic range cannot be determined!
The combination of a quiet VFO and the Q-Multiplier, yields a receiver of superior performance in the signal-to-noise ratio.
The other features of the 455 IF make over for shortcomings in single conversion designs in the present literature. They are being able to have a wide range BFO and a stable, quick working automatic gain control (AGC).
Present designs in the literature give you two options. If you buy a commercial 9 MHz IF filter, the BFO consists of two high-priced crystals and the complication of building two switched BFO's. The other alternative is to pull a crystal of the same type used in the filter. The problem becomes acute when trying to copy SSB on 40 meters and above. It is easy to lower a crystal frequency, but much harder to raise the frequency. Since 40 meters and above use upper sideband, the crystals must be raised in resonant frequency. This is sometimes difficult to do. Either way, your stuck without much range in your BFO frequency.
The BFO for a 455 kHz IF, however, is easy to implement with a single oscillator made with the same coils you use in the IF. The range can be as wide as you want. Upper and lower sidebands are as simple as tuning a knob. This also provides another tool to suppress interfering signals.
A well-behaved AGC is the most useful advantage of the 455 IF. I use a separate helically wound short antenna for receiving. The AGC works so well that I don't have to touch the receiver when I begin to transmit. Full QSK is achieved with no additional circuity or hassles. Running a QSO is as simple as with a transceiver. I am also hearing my live signal and can immediately discern any problems.
A 455 kHz IF is easily tamed. BFO back-feeding through the product detector is hardly the same as at 9 mhz. Detecting a 445 signal for your AGC control yields much higher voltages and a better acting AGC than at higher IF's. The result is a simple circuit, fewer shielding and isolation design problems, and consistent results with different building techniques employed by amateur radio homebrewers.
There was one problem - noise. Most receivers designed with 455 IF's used very high gain integrated circuits like the CA3028. By using MOSFETS, inexpensive 6 kHz wide ceramic filters, and a little less gain, the 455 IF is so quiet that at full volume with the antenna disconnected, I can't tell if my receiver is turned on!
Using a double conversion design, you can still use a crystal filter of your choosing yielding whatever passband you want to design at low cost. This also cures the image problems that plagued single conversion 455 IF designs. A crystal oscillator 455 kHz from the IF frequency is necessary to downconvert the first IF. The most common are the computer crystals of 3.457 and 4 MHz. They are widely available and work extremely well in the receiver presented in this article. Surplus Sales of Nebraska and Tanner Electronics (1100 West Valwood Pkwy., Suite 100, Carrollton, Tx 75006) have many other combinations available.
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