NOISE FIGURE METER

Black (Gain) and Magenta (Noise) with an attenuator after the amplifier.

Yellow (Gain) and Light Blue (Noise) with an attenuator before the amplifier.

One may say that the Noise Meter is going "backwards". The lower (better) NF, the larger reading. It can be seen as a "Noise Quality" rather than "Noise Figure" Meter.

The Gain Meter is normal; the more gain, the larger reading.

Two things attract attention:

First: with the attenuator after the amplifier the Noise reading is almost flat up to A=10 dB. The Gain drops right away, with increased attenuation, as it should.

Meaning: If an amplifier has a good NF in itself, and some good gain, the loss after the amplifier will not affect the NF. (But even a small amount of loss on the input to a low noise amplifier will degrade the NF by as much.)

Second: with the attenuator before the amplifier the Gain reading (Yellow) follows the Gain with the attenuator after the amplifier (Black) up to ~ A=20 dB, then it levels off! This is the noise of the amplifier itself coming through! The gain of this amplifier is rated ~20 dB. The NF drops like a rock right away. (The published NF for MAR-6 is ~2.8 dB.

There is some more work to be done in determining the interpretation and the best gain settings of the channels and of the readings.

The LogProbe Noise Figure Meter is somewhat unique in that it can measure on a receiver direct, RF in, Audio out as well as on an UHF amplifier with 900 MHz in 900 MHz out!

The NF-1 can be used to compare and trim amplifiers for best gain/noise figure. In most amplifier designs the most gain and the best NF are not found at the same setting, so a little gain is often sacrificed for a better NF. The result is often referred to as "Associated Gain", the Gain associated with tuning for the best NF.

EXAMPLE: you may have an amplifier with 20 dB gain when tuned for max gain, and 1.5 dB NF. Tuning for best NF may result in 17 dB gain and an NF of 1.0 dB. This improved NF may be quite important, whereas the reduction of gain may not even matter. Let us se why:

Say the amplifier is an antenna amplifier located by the antenna (an active GPS antenna for example) driving a receiver with a NF of 3 dB via a long cable, introducing 8 dB of loss. Without the pre-amp, but with the cable, the receiver would have a NF of 3 + 8 = 11 dB! Forget the GPS satellites! You will never even receive them!

With the pre-amp, having 17 or 20 dB gain, there is still 9 or 12 dB of gain after the cable. By far outdoing the 3 dB NF in the receiver. The system will essentially have the NF of the pre-amplifier, so its NF is important! The 0.5 dB NF improvement will improve system performance accordingly, whereas a gain of 17 or 20 dB will make no difference unless the cable loss came up to some 3 ~ 6 dB less than the pre-amplifier gain.

NF-1 can easily detect the difference between a 1.5 or a 1.0 dB NF, as well as gain variations of < 0.1 dB.

Connect the source to the input and LogProbe to the output! A series resistor may have to be employed, adjusting the 50 Ohm output of the source to what the amplifier expect s to "see." I.e. a 47 k Ohm resistor for a 47 k Ohm input. Use a metal film resistor in order not to contribute unnecessary to the noise. Different bias currents and resistor types on the input transistor may affect the gain and noise.

Beware that for to reach down to 20 Hz you would have to assemble an input plug for LogProbe, using 10 µF coupling capacitors and protection diodes. The source do not have much output below 10 kHz, so it is barely worth it. Use at least 15 nF. Maybe in the future, if the demand is sufficient, we will develop a source for the audio applications.

Let us first divide receivers in a few classes:

FM receivers

AM receivers

SSB receivers

TV receivers

The TV has an AM receiver for the picture and FM for the sound. Follow the advises below! Note that the NF of a TV "is in the tuner" and trimming it will thus affect both picture and sound. One can of course trim an IF strip by itself by injecting the signal at the strip input. The TV has two strips, one each for picture and sound. Especially the picture IF is tricky to align, so if you do not have the experience and equipment, leave it alone!

The SSB receiver is straight forward. The noise source is connected to the antenna input and LogProbe to the loudspeaker output. The added noise from the source is audible and LogProbe will pick it up. NF-1 will work. The volume control may have to be set for some reasonable reading on the Gain meter. The RF Gain may have to be turned down, used as a volume control, and the Audio Gain set to max. This way the AGC (Automatic Gain Control) of the receiver will not kick in.

The AM receiver have to be treated the same way. Audio Gain at max, RF Gain as a volume control. LogProbe to the loudspeaker. If it is a regular "BC-receiver" (Broad Cast), the normal "radio" one have for to listen to music on, the RF gain is not available for adjustment. A variable attenuator between the LogProbe and the antenna input adjusted for a very weak signal from the noise source will probably do since most AGC is "delayed", that is: it will not kick in until the signal get a little bit stronger. If this do not work, the AGC has to be disabled by shorting it to ground with a clip lead. Also notice that the antenna input is probably not 50 Ohm, so an adapter resembling the antenna impedance to be used should be employed. In general, it does not matter much anyway since the atmospheric noise will not come down to the level of a good receiver input until a few hundred MHz. At "AM", a few MHz, it is quite strong and will swamp all but the worst receiver inputs!

The FM BC receiver. In a FM receiver the gain in the IF section is (or should be) so high that limitation of the signal occurs in, or before, the detector. This is usually not the case in BC radios, since the radio would be very noisy between stations, and the non-technical consumer neither understands or likes this. The detector do not detect amplitude anyway, so there will be no contribution from the noise source on the audio output. Therefore LogProbe has to "sniff off" amplitude of the the IF (usually 10.7 MHz) before the detector. The collector, or anode, of the last IF amplifier will carry this signal. Sometimes it can even be picked up with a little coil from the trim-hole in the last IF transformer. In radios where the IF section and the detection is done in one IC, quite usual nowadays, the RF signal into this IC, from the mixer or filter preceding it, is the one to monitor with LogProbe. A small coupling capacitor from one of its inputs to the input pin of the IC will do. Keep it short or you invite oscillations! The RF stage can now be tuned for the best Gain/NF compromise…

The FM communications receiver may very well have an IC like described above. If it has tubes or transistors in a lumped IF section, the gain is probably high enough that limiting already has taken place in the last IF stage. These receivers are "shut up" with a squelch control to make them quiet for no input signal. Open the squelch and tap the IF stages far enough back (towards the mixer) from the detector that you get a signal that is still not limited! If you read gain but no NF, the signal is limited. Or the RF amplifier is dead.

Over voltage on the LogProbe input from using a blank wire touching something with > 5V.

Over voltage on the LogProbe input from a strong loudspeaker output signal!

Be careful not to exceed 0 < V_IN < 5 V on the LogProbe input, or I will have to sell you a new one!

The NF-1 includes a wall plug adapter for 115 or 230 V AC in and 12V DC out. The instrument will have two analog meters on the front panel, probably with sensitivity controls.

NF-1 includes a noise source with about 13 dB ENR, good to > 2GHz. Source RL is better than 30 dB. Output connector SMA-F or BNC-F.

NF-1 will be fully compatible with the later LogProbe Goldä , good to > 2.5 GHz, to be introduced next year.

NF-1 is $750 (+ 7.25% tax for CA residents, $54.38.) The LogProbe is $250, is not included with the NF-1 .

A LogProbe and the NF-1 will contain all it takes to get going!

For mail or orders, please contact us at