If you have ever listened to the weak signal modes on the LOWfer / MEDfer / HIfer bands or used the Exalted Carrier Single Sideband (ECSS) technique for weak signal broadcast listening, you know how important it is for the receiver to be exactly on the proper frequency. Sound quality is best, and the chances of picking up a weak beacon are maximized when the radio is accurately tuned. I use narrow DSP filters to get NDB identifiers and also have fun with modes like QRSS (very very slow CW), and can reliably tune to within a few Hertz after using the alignment procedure below.
The following is a procedure for aligning the PLL and beat frequency oscillators in the ATS-909 / DX-398 receiver, and assumes there is good reception of WWV on 10 MHz and an audible broadcast station on one of the channels near 1000 kHz. Other broadcast band channels may be substituted...I sometimes use WLS on 890 kHz since it is so strong here in the midwest. Audio spectrum analysis software, such as Spectran or ARGO, should be used to measure the audio beat frequencies.
The 4332.0 kHz PLL oscillator is adjusted by TC301, and any tuning error will cause a proportional error in the received signal. A 55.395 MHz oscillator converts the first IF to the second IF, and is adjusted by transformer T14, and any error will cause a direct Hertz for Hertz shift in the received signal. The BFO operates at 448.0 kHz when receiving lower sideband, and is adjusted via trimmer VC5. When receiving upper sideband, 452.0 kHz is the BFO frequency, and it is adjusted by trimmer capacitor VC4.
|PLL Reference Oscillator||4.332 MHZ||TC301|
|LSB BFO||448 kHz||VC5|
|USB BFO||452 kHz||VC4|
Start up Spectran or Argo, and feed audio from the '909 into your computer sound card. You should see a display of the received audio spectrum. Open the radio, go to the front half of the radio, and remove the screws holding down the keyboard and display circuit board. Carefully undo the plastic tabs and tilt the board up just enough to access T14 and the BFO trimmers.
1. Turn on the radio and tune in WWV on 10 MHz in AM mode.
2. Switch to USB and adjust the trimmer TC301 for a zero beat. Wait two or three minutes and readjust if necessary.
3. Tune to a local broadcast station close to 900 kHz in LSB mode. Use the SLOW tuning steps to zero beat the station.
3. Tune to a local broadcast station close to 900 kHz in LSB mode. Use the SLOW tuning steps to zero beat the station. Then switch to FAST steps. Rotate the main tuning knob to 897 kHz, and listen for BFO leakage. The spectrum display should indicate a carrier near 1000 Hz. Adjust VC5 to set the tone at exactly 1000 Hz. Wait two or three minutes and readjust if necessary.
4. Tune to a local broadcast station close to 900 kHz in USB mode. Use the SLOW tuning steps to zero beat the station. Then switch to FAST steps. Tune to 903 kHz, and listen for BFO leakage. The spectrum display should indicate a carrier near 1000 Hz. Adjust VC to set the tone at exactly 1000 Hz. Wait two or three minutes and readjust if necessary.
5. Tune to a local broadcast station close to 1000 kHz in AM mode. Switch to USB, then step down 1 kHz using the FAST step setting. Using the local broadcast station's carrier, measure the beat note. It should be near 1000 Hz (error = 1000 Hz minus the beat frequency).
6. Adjust T14 for a 1000 Hz note. Wait two or three minutes and readjust if necessary.
7. Go to 9999 kHz USB, note the beat frequency, then adjust TC301 to set a 1000 Hz tone. Wait two or three minutes and readjust if necessary.
8. Repeat steps 5 through 7 above until the beat note stays at 1000 Hz, indicating that the oscillators are properly aligned. Each repetition of this bracketing procedure will reduce the oscillator errors, until the error is negligible. There should be a 1000 Hz tone when tuned 1000 Hz below WWV or any HF time and frequency station.
9. Reassemble the radio. After an hour operating reassembled, TC301 may need slight readjustment due to thermal drift. Re-alignment should not be necessary for several months to a year after completing this procedure.
Happy Tuning, Folks!