METER ZERO check:

• CARRIER and AUDIO controls to MIN (counterclockwise - zero carrier and zero modulation).
• METER READS to CARRIER-CW (MOD XTAL & METER SELECTOR).
• The meter should read zero after a 15-min warm-up period; if not, adjust by turning CARRIER METER ZERO using a screwdriver.
• Set METER READS to %MOD-400/1000 (MOD XTAL & METER SELECTOR), the meter also should read zero.

The elusive topic of IMPEDANCE BALANCE in the OUTPUT CABLE:

The generator's signal, after passing through the attenuator, must continue along the output cable to its application at the test point. This cable is an RF transmission line with the same characteristics as a transmission line to an antenna; therefore, the impedance at the generator's output point must match the impedance at the test input point. If this does not happen, the voltage measurements will be inaccurate due to standing waves in the connecting cable. The URM-25D must "see" a 50 ohm load at the point of application of the output cable, which is the same impedance as its intput point.

This terminal resistance makes the output line or cable APERIODIC (independent of the frequency passing through it), eliminating standing waves and making both the input and output of the cable purely resistive. The cable length is irrelevant to the frequency as long as the impedance remains constant; however, the length does have a slight influence, since the pure resistance of the cable will cause some signal attenuation. See below for a drastic application of this principle; accessories already contain the required load resistance, eliminating the need to connect an impedance adapter + accessory via a short cable.

OUTPUT TERMINATION & signal application:

Unless the output cable is properly terminated, standing waves will develop in it. As the frequency increases, the physical length of the output cable becomes a substantial fraction of the operating wavelength. This cable will act as a parallel resonant circuit at some frequencies and as a series resonant circuit at other frequencies. When connecting an ohmmeter to BNC RF OUT X MULT to measure each attenuator output position, all values should be approximately 50 ohms, and proper termination of the output cable using the MX-1487 impedance adapter makes its impedance purely resistive at all frequencies.

A properly terminated coaxial output cable connected to RF OUTPUT X-MULT will allow the URM-25 to operate without standing waves present. Impedance adapters and accessories must be next to the device under test, not next to the URM-25. There are two different connection sequences:

1) Connection sequence for attenuators:
RF OUTPUT X-MULT BNC + long 50 ohm coaxial cable + [Signal ATTENUATOR 5:1 / 10:1] + short coaxial cable or dual female BNC connector + [MX-1487 impedance ADAPTER].

2) Connection sequence for accessories:
RF OUTPUT X-MULT BNC + long 50 ohm coaxial cable + [MX-1487 impedance ADAPTER] + short coaxial cable or dual female BNC connector + [ACCESSORY CX-1363/U / SM-35].

As the impedance adapter should always be there to terminate the long 50 ohm coaxial cable (combining a 50 ohm source and a 50 ohm load to ensure a calibrated voltage), the last two steps in the sequence above may be combined into a single adapter.

Combining impedance adapter + accessory in a single unit: MX-1487 + CX-1363/U & MX-1487 + SM-35. The CX-1363/U test lead accessory prevents any voltage from the device under test from entering the signal generator attenuator and burning out a calibrated resistor to ground within it. The SM-35 dummy antenna accessory (antenna simulator) prevents the signal generator from loading the receiver's antenna input by excessive coupling due to the generator's low output impedance.

METER READINGS & output VOLTAGES:

• A frequency setting with MICROVOLTS all the way to the right and CARRIER CW set to "10" means:
• 2 Volts at BNC X-200K RF OUT, "X-200K" is a multiplication constant, e.g. 10 µV end scale x 200,000 = 2 volts.
• 0.1 Volts at BNC RF OUTPUT X-MULT with RF MULTIPLIER at X10K (10 µVolts x 10,000 = 100,000 µVolts = 0.1 Volts).
• Using RF OUTPUT X-200K, the output voltage in µVolts is determined by multiplying the meter reading by 200,000.
• Using RF OUTPUT X-MULT, the output voltage in µVolts is determined by multiplying the meter reading by the number that RF MULTIPLIER points out (.1, 1, 10, 100, 1000, 10000). Actual calibrated output values are only achieved when a 50 ohm load is connected to RF OUTPUT X-MULT, (that's why the custom test lead above was made inside an impedance adapter; otherwise, the test lead accessory must be connected after an impedance adapter in order to properly terminate RF OUTPUT X-MULT into 50 ohms).
• Meter range is 0 to 10 µVolts; with the RF OUTPUT X-MULT BNC terminated in 50 ohms, the maximum calibrated voltage (meter → 10) is 0.1 Volts with RF MULTIPLIER to X10K (10 µV x 10,000), and the minimum calibrated voltage (meter → 1) is 0.1 µVolts with RF MULTIPLIER to X.1 (1 µV x .1). Thus, the calibrated output voltage range is from 0.1 µVolts to 0.1 Volts.

Having all this clear, 90% on precision use of the URM-25 is covered. That calibrated voltage range is usually enough, so it is not necessary to have all the accessories of above, the most recommended of them is the test lead (can be easily DIY made) that isolates the B+ of the device under analysis protecting the output circuit of the generator. The test lead has two capacitors in parallel with capacity values that respectively adapt 10KC-300KC ("high-audio") and 300KC-50MC ("radio") frequency responses (see schematics page); both capacitors are in series with the active test lead and also isolates the device under analysis from a possible B+ leak from the generator. URM-25 advanced uses can be achieved by reading the manual, which is available at BAMA (click on NAVSHIPS 0967-187-5010).