what rx relative signal db to use in radio mobile
| Up 1 level Back to Habitation | Receiver Sensitivity Measuring Methods By Robert W. Meister WA1MIK |  |
There seems to exist 2 mutual methods of measuring FM receiver sensitivity: 20 dB quieting, and 12 dB SINAD (Indicate plus Noise And Distortion). Most people similar one method or the other, and strongly defend their choice. This article will examine the equipment and procedures I used, and compare the results.
Dorsum in the "good old days" when FM radio was wide-band (15 kHz deviation or more than), the 20 dB quieting method was used almost oft. Receivers needed to be accurately and thoroughly tuned. The complete IF (Intermediate Frequency) section could exist aligned, and the service manuals really gave you full and consummate procedures to arrange everything in the radio. That level of item doesn't exist any more. Many radios are considered to be throw-abroad items. The factory does the alignment, manuals are less and less detailed, and what adjustments are present, are washed with software. Some people would wait upon this as progress.
With the level of integration present in modern communications equipment, receivers hear things a lot better and tend to have broad-band front ends, but the average person can no longer marshal them.
At that place are several other practiced articles about measuring and understanding FM receiver sensitivity elsewhere on the Repeater-Architect spider web site.
Precautions:
Near examination equipment measures signals relative to ground, and one of the input lines is very likely to exist at basis potential. Be careful when attaching things to the radio's loudspeaker lines. On a radio such as a Motrac, Motran or MICOR, ane side of the speaker is grounded. On a MaxTrac 1 side of the internal speaker is connected to ground through a 1 ohm resistor. On a Spectra, Mitrek, or GM300 mobile neither side of the loudspeaker tin ever be grounded or you volition allow out the audio amplifier's lifetime supply of magic smoke. If y'all are testing a non-grounded speaker output you need to use an audio isolation transformer. You can become one from Motorola (part number 2580188B01), from a high-end automobile stereo shop, or from web/mail order suppliers like Crutchfield. Just tell them you demand an 8-ohm to viii-ohm isolation transformer that tin can handle about 2 to 3 watts.
The radios that preceded the Mitrek in the Motorola mobile product line mostly used push-pull audio PA decks and audio output transformers, and drove the speakers via a hot wire to basis. The Mitreks and many later on radios eliminated the sound transformer and they run the speaker as the button-pull load directly... both sides floating off of footing. This quirk is significant on your workbench: any audio exam equipment that connects to the speaker (such as you lot would use to make a quieting measurement) must exist on the far side of a three.two ohm or viii ohm 1:1 audio transformer. Motorola has a low-power i equally part number SLN6435 in their examination equipment catalog, and they too include a higher ability one in every tabletop base station (part number shown in a higher place) since common wire-line remote controls expect basis-referenced audio to drive the remote speakers. You can order either one as a spare part from Motorola.
Sensitivity-measuring Equipment:
I used an Agilent E4430B synthesized 1 GHz indicate generator as the source of RF. The modulation was the internal 1000 Hz sine wave oscillator set for 3.0 kHz FM difference.
My indicating instrument for twenty dB quieting was a Fluke 189 true RMS digital multimeter. This meter has the ability to brandish Air conditioning millivolts and dBm at the same fourth dimension. The only downside to a DMM is that the update rate tends to make the displayed value jump effectually when dissonance is a significant part of the input signal. I also used the AC voltmeter on the HP 334A distortion meter, and the two were within 0.5 dB on all readings.
I used an HP 8901B Modulation Analyzer to mensurate SINAD besides equally distortion. It has a digital brandish but can only measure SINAD or distortion at 400 Hz or 1000 Hz. The baloney tin be viewed every bit a percentage or dB below the reference level (negative values). The SINAD function merely shows the absolute value of the distortion's dB values.
I used an HP 334A Distortion Meter for the alternate SINAD method. This unit of measurement covers 5 Hz thru 600 kHz and can measure baloney well below 0.1% (more than 60 dB down). Other HP models such equally the 331A, 332A, and 333A can also be used. Baloney tin can be viewed equally a percentage or dB below the reference level.
I had the chance to infringe an IFR 1200SS service monitor to measure SINAD. This particular unit has both an analog meter and a digital readout and measures directly in dB. At thirty dB SINAD signal level, this unit of measurement indicated 3.2% distortion and my HP 334A measured iii.4%. I'd consider that "close enough for government work."
I tend to use dBm for my RF point levels. If you lot prefer microvolts, a handy two-page conversion tabular array can be plant in this 72kb PDF file
How SINAD Works:
A SINAD meter, or role in a service monitor, consists of an Ac voltmeter and a narrow audio filter tuned to 1000 Hz. Some units tin likewise generate their ain tone. An RF signal modulated with a 1000 Hz tone is fed into the receiver. The detected sound level is measured (SIgnal plus Noise And Distortion). The filter removes the thou Hz signal, and the resulting audio is then measured (Noise And Baloney); the difference between the ii values is the SINAD number in dB. Some units may utilise an automatic gain command to set the reference level; others may rely on two meters or a switch to mensurate the pre-filter and mail-filter levels; still others simply use a differential voltmeter excursion to make up one's mind the difference between the 2 levels. Whichever method is used, the difference between the signal levels with and without the 1000 Hz tone filtered out gives you lot the dB SINAD value.
SINAD measurements take into account any non-linear operation, irregularities, or deficiencies in the IF, detector, and audio stages of the receiver, mainly considering these can all increase the distortion present in the audio indicate. It likewise requires that the modulating equipment and signal have a lower distortion level than the receiver you lot're trying to measure. Tuning a receiver for all-time SINAD value may take longer and require more equipment, but information technology volition also get the most performance out of the entire system.
It is critical that the one thousand Hz filter and the 1000 Hz tone exist accurately tuned to each other. Many SINAD meters requite you the ability to adjust ane or the other. If your SINAD instrument generates its own tone, feed that into the meter's input and adjust the filter tuning for the lowest meter indication (best SINAD value). The absolute tone frequency is not important (within 1%) as long equally the notch filter is tuned to it.
twenty dB Quieting Method:
I've ever used the 20 dB quieting method for measuring sensitivity, but non for the absolute "mill spec" value. I tend to use information technology for comparison with other similar receivers. My reason for liking this method is simple: it requires very piddling test equipment, and even that doesn't take to be very sophisticated.
Connect a betoken generator to the receiver's RF input port, and an Air-conditioning voltmeter to the loudspeaker. On a MaxTrac, I employ the handset audio signal present at the forepart panel MIC jack. The official Eia specification says to utilise the speaker terminals. Whichever is used must provide de-emphasized audio.
With no signal present, open the squelch and measure the Air-conditioning voltage. If your AC meter has a dB scale, annotation that reading. On a 450 MHz MaxTrac, I usually measure nearly 700 millivolts or -ane dBm (relative to ane milliwatt at 600 ohms) on racket.
Transport an united nations-modulated RF signal into the radio, starting at the everyman level possible. As you increase this signal level, the receiver noise will start to decrease. This is the "quieting" effect that FM is and so well known for. Conform the RF level until the Air-conditioning voltmeter reads 10% of, or 20 dB lower than, the value you had with no signal. The amplitude of the RF signal is now the 20 dB quieting sensitivity of the receiver. On a 450 MHz MaxTrac, the Air conditioning voltage would now be effectually 70 millivolts or -21 dBm, and the sensitivity is usually effectually -118 dBm (just under 0.3 microvolts).
You can often measure out the 20dB Quieting sensitivity just past listening to the received sound on the radio's loudspeaker. Every bit you increase the RF level, the racket will decrease, and and so will the crackling and popping sounds. When you get to the indicate that you hear one pop or crackle every few seconds, that's usually the 20-22 dB Quieting betoken. Y'all can apply this to rapidly compare receivers or preamp functioning.
12 dB SINAD Method:
Connect a point generator to the receiver's RF input port, and a service monitor (or other piece of equipment capable of making SINAD measurements) to the loudspeaker. On a MaxTrac, I use the handset audio signal present at the forepart panel MIC jack. The official Environmental impact assessment specification says to use the speaker terminals. Whichever is used must provide de-emphasized audio.
Fix the RF betoken source for a strong signal modulated with a 1 kHz tone set up for 3 kHz deviation (60% of the receiver's normal bandwidth). For ii.5 kHz broad systems, you would set up the deviation to 1.5 kHz. If your SINAD meter can generate its own tone, use that as a modulating source. Arrange the brandish device for a reference level if necessary.
Start decreasing the RF signal level. The receiver noise will simultaneously increase. Continue lowering the signal until the meter reads 12 dB (or 25%). The amplitude of the RF signal is now the 12 dB SINAD sensitivity of the receiver. On a 450 MHz MaxTrac, the 12 dB SINAD sensitivity is usually around -120 dBm (about 0.22 microvolts).
Alternating SINAD Method:
If you don't take a SINAD meter or a service monitor with that capability, you can make a like measurement with an sound distortion meter. This device consists of a tunable notch filter and an Air conditioning voltmeter that can measure out the sound level with or without the filter in the circuit. By notching out the fundamental audio tone, the residual racket and distortion can be measured. Note that this is exactly what a SINAD meter does. Not surprisingly, the procedure is most identical.
Connect a signal generator to the receiver'south RF input port, and an audio baloney meter to the loudspeaker or handset output, only similar yous would exercise with a SINAD meter.
Set the RF betoken source for a strong signal modulated with a 1 kHz tone gear up for three kHz deviation (60% of the receiver'south normal bandwidth). For 2.five kHz wide systems, y'all would set the deviation to one.five kHz. Adjust the distortion meter for its reference level of either 0 dB or 100%, then conform the tuning for minimum distortion level. It should be possible to get the distortion beneath 3% (thirty dB down).
Kickoff decreasing the RF betoken level. The receiver noise will simultaneously increase. Go along reducing the bespeak until the meter reads 25% baloney (12 dB downwards). The amplitude of the RF betoken is at present the 12 dB SINAD sensitivity of the receiver. On a 450 MHz MaxTrac, the SINAD sensitivity is ordinarily around -120 dBm (around 0.22 microvolts).
Receiver Performance:
I used a 450 MHz MaxTrac equally my subject area receiver considering information technology was the nearest radio to my test bench. I used the handset output signal on the MIC jack. Technically, I should have continued my equipment to the loudspeaker terminals, merely since the handset jack is the detected and muted audio at the acme of the volume control, the but circuitry that was not included was the audio power amplifier.
I did try several measurements with my equipment connected to the speaker terminals. This had a very slight effect on the distortion, but it was negligible, under 5%. I therefore stand behind my decision to use the handset output on the MaxTrac.
I measured the baloney at several audio frequencies at both three kHz and 5 kHz departure. I found that the lowest distortion was obtained with an RF level of -91 dBm, and then I left it in that location. I besides plant that slight tuning (+/- ii kHz) of the carrier frequency fabricated a drastic deviation on distortion at the higher deviation level.
| Audio Freq. | % Baloney | |
| 3 kHz | 5 kHz | |
| 250 | ane.0 | 1.5 |
| 500 | 0.5 | 0.8 |
| g | 1.1 | 1.v |
| 1500 | 1.3 | one.3 |
| 2000 | 1.3 | ane.2 |
| 2500 | 1.eight | 1.2 |
| 3000 | two.3 | 1.5 |
I likewise measured the distortion while adjusting the RF frequency up and down past as much as vii kHz. This table shows that at that place is a slight imbalance in the IF tuning on this receiver.
| RF Freq. Error | Modulating Frequency | |||
| 400 Hz | grand Hz | |||
| + | - | + | - | |
| 0 kHz | 1.three | ane.iii | 1.0 | ane.0 |
| 1 kHz | one.vi | 1.3 | 1.5 | ane.7 |
| 2 kHz | ii.ii | one.5 | 3.0 | 2.4 |
| 3 kHz | 2.6 | 1.5 | 3.5 | 2.5 |
| iv kHz | 2.v | 1.7 | 3.four | 3.0 |
| five kHz | 2.i | 1.7 | 3.4 | 3.6 |
| vi kHz | ane.8 | 1.vi | iv.v | 4.8 |
| vii kHz | 1.8 | 1.iii | v.0 | 6.0 |
I varied the carrier frequency at 2 dissimilar signal levels (twenty dB quieting and xxx dB quieting) to see how much of an effect that had on the quieting measurement. The values in the tabular array are dB Quieting:
| RF Freq. Error | Quieting | |||
| 20 dB | 30 dB | |||
| + | - | + | - | |
| 0 kHz | -twenty | -20 | -30 | -30 |
| ane kHz | -20 | -19 | -29 | -30 |
| 2 kHz | -xix | -18 | -29 | -30 |
| 3 kHz | -18 | -16 | -29 | -30 |
| 4 kHz | -14 | -12 | -30 | -30 |
| 5 kHz | -9 | -6 | -30 | -29 |
| 6 kHz | -iv | -one | -30 | -27 |
| 7 kHz | -ane | -ane | -20 | -16 |
I measured the diverse quieting and SINAD levels and summarized them below. A 1 kHz sound tone at iii kHz difference was used for all the tests except quieting. All negative values are the RF signal level in dBm required to reach the specified dB quieting or SINAD level. The column headings list the equipment model number used to measure out Quieting or SINAD, equally described in the Sensitivity-measuring Equipment section to a higher place.
| dB [1] | 189 | 8901B | 334A | 1200SS |
| 3 | -127 | -125 | -125 | -124 |
| 6 | -125 | -123 | -123 | -122 |
| 9 | -123 | -121 | -122 | -121 |
| 12 | -122 | -120 | -121 | -120 |
| 15 | -120 | -119 | -120 | -119 |
| xviii | -119 | -118 | -119 | -118 |
| 21 | -118 | -117 | -117 | -117 |
| 24 | -116 | -115 | -115 | -116 |
| 27 | -113 | -112 | -112 | -113 |
| 30 | -110 | -108 | -108 | -110 |
| 33 | -107 | -103 | -105 | -106 |
| 36 | -104 | -99 | -100 | -102 |
| 39 | -101 | -91 | -91 | -91 |
Summary of Measurements and Conclusions:
I found very petty difference betwixt the readings I got with the SINAD and the baloney meter. The values were and so close, the variations could be due to equipment temperature and rounding errors (I only recorded signal levels in whole dBm values). The numbers I got with the 20 dB quieting measurement were also extremely close to the SINAD/Baloney values, certainly within +/- 1 dB of each other. It was squeamish to see that the distortion and SINAD values also matched the recently calibrated IFR service monitor.
The 20 dB quieting values showed less sensitivity to signals beingness off frequency. This method too did not accept into account any bug in the IF, detector, or sound stages of the receiver. In fact, if the audio amplifier had a lot of distortion with tones, this measurement would not have shown that.
The SINAD and baloney values were very sensitive to the carrier frequency. Slight errors here were immediately detected by the equipment. Any problems with the sound stages would have reduced the SINAD values, requiring a college RF indicate to compensate.
It was never my intention to forcefulness any method of measuring sensitivity upon the reader. I only wanted to prove the various techniques and compare the results. It would appear that one could measure absolute sensitivity using whatever of the test procedures and equipment mentioned in this commodity with fairly authentic results. For true receiver performance, the SINAD method takes into account more of the radio's circuitry, just if all y'all want to do is compare sensitivity, for example, with and without a preamp, the xx dB quieting method is simply as reliable. Of course, if the preamp adds a lot of racket to the received betoken, the quieting measurement could be affected and requite a false reading. This would take a similar worsening effect on the SINAD value equally well. I even so like the 20 dB quieting method because I detect information technology easier to determine past ear: when I've got enough indicate to eliminate all the audible crackles, that's 20 dB quieting. Besides, 12 dB SINAD is a adequately noisy signal and rather touchy; i dB of RF input level change can make a 3 dB departure in the SINAD value.
Audio Samples:
I recorded 6 files, all at 11 kilobits per second (8-bit mono) sampling rate, at several RF input signal levels, using the handset output pin of the MIC connector on my 450 MHz MaxTrac. Each file contains about five seconds with no signal, followed by near five seconds with a signal. These are all .WAV files about 115kb in size. When you click on a file information technology will be downloaded to your system and whatsoever application your reckoner is configured to utilise to play .WAV files volition open up and play them. As we (the repeater-builder.com staff and myself) don't know your computer, we tin can't help you if they don't work. Before this page was made public nosotros had Windows, Macintosh and Linux systems examination the prototype page and the files work on all of them without error.
12 dB Quieting: -122 dBm
12 dB SINAD: -120 dBm
20 dB Quieting: -118 dBm
xx dB SINAD: -118 dBm
30 dB Quieting: -109 dBm
30 dB SINAD: -109 dBm
Credits and Acknowledgements:
Thank you get to Rob Barba for driving 30 minutes each style to my house with his IFR 1200SS service monitor i evening so I could spend 10 minutes acquiring information for this article.
Cheers too go to the various people who read the article and offered constructive comments.
Of grade, thanks ever become to Mike Morris WA6ILQ of the Repeater-Builder staff for converting this commodity to HTML.
Contact Information:
The author tin can exist contacted at: his-callsign [ at ] comcast [ dot ] internet.
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Article text © Copyright 2006 by Robert West. Meister WA1MIK
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