When tuning a receiver to a broadcast station that transmits all the time you just listen for the station and tune for best signal. But that can not be done for utility or military stations that transmit only for a short time. For these you need a tuning accuracy that is precise enough to center the transmitted signal in the radio's receive bandwidth. For a CW signal this is on the order of tens of cycles.
This type of frequency meter was made during the time of analog radios where there was a tuning dial on receivers whose accuracy was poor. By that I mean that if you tuned to the frequency of a station that was not then transmitting and waited for a transmission you may not hear the station when it did transmit because the calibration error of the radio was greater than the bandwidth of most stations. So there needed to be an accurate tune the receiver as well as the transmitter.
One way was with a crystal calibrator. I have a Heathkit HD-20 Crystal Calibrator that uses a 100 kHz crystal and so had a harmonic every 100 kHz. This would allow calibrating the receiver to the nearest 100 kHz which might cover 10 stations. This was just a small box with an on/off switch and a terminal for a short wire antenna that would allow coupling the signal to a receiver. You could set the receiver dial so it was right on one of the harmonics then the dial accuracy would be improved. Note after setting the dial an some harmonic the error would increase away from that frequency so that if you wanted to receive at some other frequency a few MHz away the dial would no longer be accurate.
The early "frequency meters" were not frequency meters in the sense that they measure frequency. See the HP Memory Project for very early test instruments the measure frequency and my HP AC4A tube type decade counter web page for more on that. Instead they were precision oscillators that could be used to calibrate a receiver, or by listening to a beat note using headphones compare the oscillator output to the signal of another oscillator, like from a transmitter that needed to be set to a precise frequency.
As part of my general interest in precision time and frequency now and then I considered buying a BC-221 Heterodyne Frequency Meter. These were W.W.II vintage precision oscillators covering the 125 kc to 20 Mc used so that a receiver or transmitter could be tuned to the assigned frequency. The thing that's interesting about the BC-221 is that it's calibration book was custom made for each serial number frequency meter. There was (is?) a web page describing the automated way that each calibration booklet was made using semi automated equipment. That was needed because analog radio equipment was not accurate enough.
With the advent of synthesized radios based on a stable crystal reference oscillator the need for frequency meters declined. Although they were still in service to support the older analog radios for many decades.
Heterodyne Frequency Meters
BC-221 (part of the SCR-211) TM 11-300
LM series for example the LM-13: 195 kHz - 4 MHz
TS-series for example the TS-323: 20 - 450 MHz
Navy nomenclature (a bunch of letters - a long number
From the TM: "Frequency Meters AN/USM-159, AN/USM159A, and AN/USM-159B (fig. 1-1) are precision instruments used to measure radio frequencies (RF) in the range from 125 kilohertz (kHz) to 1,000 megahertz (MHz). They can also be used as signal generators to provide modulated or unmodulated signals for testing and calibrating radio equipment. A system of checkpoints is provided to check the calibration of the instrument throughout its operating range." 0.01% accuracy.
This frequency meter works just like the BC-221. It includes an internal crystal calibrator with crystals at 1.0 and 2.5 MHz. The key differences with respect to the BC-221 are that instead of a calibration book there's a very long 35mm film. The film is custom made for each serial number (Ref ER mag Nov 2015) . The manual has a procedure for changing the filmstrip and there is a spare strip in the instrument. There is a drawer on the front panel with "calibration tables" that seem to be generic. That implies that the VFO on each serial number is very repeatable. There are three frequency bands (125 kHz - 2.5 MHz, 2.5 - 65 MHz and 65 to 1,000 MHz) and there are three matching bands on the film so the effective length of the tuning scale is three times the physical film length times the power of the magnifying glass. This might be hundreds of feet. Note the FRR-21 LF Receiver 145-600 kHz and the SRR-13 HF Receiver 2-32 MHz use a similar optical tape tuning mechanism to get a more precise tuning adjustment. The other key difference is that instead of the tubes used in prior frequency meters this one used transistors (but no integrated circuits).
The frequency range is covered making use of both fundamental and harmonics of the internal oscillator. The table below is based on simple harmonics, but the book needs to be checked to see which harmonics are selected for which outputs (i.e. are only odd harmonics used or all of them?)
0.125 - 2.5
0.125 - 0.250
0.25 - 0.5
0.375 - 0.75
0.5 - 1.0
0.625 - 1.25
0.75 - 1.5
0.875 - 1.75
1.0 - 2.0
1.125 - 2.25
1.25 - 2.5
2.5 - 65
2.5 - 5.0
5 - 10
7.5 - 15
10 - 20
12.5 - 25.0
15 - 30
17.5 - 35
20 - 40
22.5 - 45
25 - 50
27.5 - 55
30 - 60
32.5 - 65
65 - 1000
65 - 130
130 - 260
195 - 390
260 - 520
325 - 650
390 - 780
455 - 910
520 - 1040
The CHK function was used with headphones by setting the frequency dial to a calibration point (triangle) and the Correction dial turned to get a zero beat. Then the function was switched to Operate. There are two plots for each of the bands, one showing the frequency of the calibration and the other a wide sweep showing the harmonics.
These plots were made with the frequency dial set at the same place for each band, but different places band to band. The harmonics are spaced closest together when the frequency is near the bottom of the range. As the frequency moves up the separation also increases.
On band 3 (65 - 1000 MHz) the calibration had no audio tone at 65 MHz, but did slightly higher, maybe needs a tweak? So used the calibration point at 100 MHz, but the audio tone was not clean but instead had a fuzzy sound. That's reflected in the broad noisy peak on the 100 MHz top plot below. Maybe there's something wrong with the band 3 oscillator?
0.125 - 2.5MHz
205 - 65 MHz
65 - 1000 MHz
The measured freq is 129.9975 v. 130 kHz
delta = 1.9E-8
measured freq 2.5000375
delta = 0.015 % (1.5E-4)
measured 99.9994375 (noisy)
delta = 5.6E-6
Error in marker frequency on wide display.
Correct in narrow span plot above.
Spectrum Analyzer limited to 500 Mhz max
so higher harmonics not shown
TM 11-6625-486-14&P Operator's, Organizational, Direct Support and General Support Maintenance Manual (Including Repair Parts and Special Tools) for
Frequency Meters AN/USM-159, AN/USM-159A, and AN/USM-`159B (NSN 6625-00-892-5360), 31 Oct 1975 w/Change 2 30 March 2006 216 pages
TB 9-6625-2120-35 Calibration Procedure for Frequency Meter, AN/USM-159, AN/USM-159A, AN/USM-159B
Fig 1 Carry case & Lid with accessories
Fig 2 Open Carry Case
Fig 3 Front Panel w/Cal Book
Fig 4 Front panel in Operation.
H-216/U headphones (short 1/4"PJ-054R plug)
CD-307A 4' audio ext cord
CX-12005/USM-159 AC pwr cord
Fig 5 Closeup Film strip set near 5 .000 MHz
Time & Frequency -
HP 5060A Cesium Beam Frequency Standard
HP5100 5110A Synthesizer Driver & 5100A Frequency Synthesizer
FTS4060 & Datum 4065B Cesium Time and Frequency Standard
HP 5216A 12.5 MHz Nixie Tube Electronic Counter
HP5342 Microwave Frequency Counter
TandFTE Time & Frequency Test Equp:
Gibbs Crystal Oscillator
Stanford Research SC10 Crystal Oscillator
Stanford Research PRS10 Disiplined Rubudium Oscillator
HP 53132A Counter
Stanford Research SR620 Counter
HP 33120A Function ARB Generator
HP 8648A Signal Generator
Stanford Research DG535
HP 54501 Scope
Electric Radio Magazine (ER# 310 & ERE# 311) March & April 2015: The FR-149/USM-159 Heterodyne Frequency Meter, Part 1 & Part 2
Electronics magazine May 1944 - Automatic Calibrator for Frequency Meters by David Sunstein & Joseph Tellier (pdf page 108).
PRC68, Alphanumeric Index of Web pages, Contact, Products for Sale
Page created 23May 2016