Batboard

I worked with another agency today to resolve a long standing problem they had (past tense now), and it revolved around their understanding of the level settings in 4W E&M cards. They thought the values entered for tx and rx were for gain, and I had to prove to them the values set attenuation. In other words, setting them for 0 & 0 at both ends will yield a lossless system - you'll get the same level out as you put in. If you inject -10db, you get -10db. If you set Tensr tx level for -2db and inject -10db, you'll get -8db at the far end. If you set it for +2db, you'll get -12db.

Does that agree with others experience?

Bill_G wrote:I worked with another agency today to resolve a long standing problem they had (past tense now), and it revolved around their understanding of the level settings in 4W E&M cards. They thought the values entered for tx and rx were for gain, and I had to prove to them the values set attenuation. In other words, setting them for 0 & 0 at both ends will yield a lossless system - you'll get the same level out as you put in. If you inject -10db, you get -10db. If you set Tensr tx level for -2db and inject -10db, you'll get -8db at the far end. If you set it for +2db, you'll get -12db.

Does that agree with others experience?

Almost. I have been playing with 8118 (E&M) Premisys cards, and when we set them up on the bench (a pair of channel banks running through the TDM ports on a microwave on the bench). We found that the settings of 0 / 0 for 0 db still
left us with 6 db of gain. In order to make the bi directional paths equal 0 db, I had to set the rx (on each end's receve level) to be -6.2 db for the most part. I also checked to see what the termination resistance of the port was, and confirmed it at 600 ohms. I suppose that if you were running this through a lot of in house cabling, this gain would help overcome the over all losses.
That is what I came up with. When I set the rx to the -6db (at each end), I was able to get a lossless path. I agree with your statement - the setting sets either gain or loss, depending on what you need.

B.

wa6ylb wrote:I have been playing with 8118 (E&M) Premisys cards, and when we set them up on the bench (a pair of channel banks running through the TDM ports on a microwave on the bench). We found that the settings of 0 / 0 for 0 db still left us with 6 db of gain. .

Was your external equipment that you had connected also terminated at 600 ohms? Have have seen exactly the 6 dB difference that you mentioned only to find that my device was set to high impedance. When set to 600 ohms, it dropped down by 6 dB and I had my 0 gain.

I just looked at the Tensr manual and it looks like there is 2 different points that you can set level gain/loss.
There is RX TLP and TX TLP. It looks like the gain/loss operates differently between them.

On RX TPL, you put in a positive value to increase gain, but in the Tx TLP, you put in a negative value to increase gain.

Strange.


Here is the description from the manual.

Rx TLP
The Receive Transmission Level Point (TLP) setting controls the amount of gain or loss added
to a signal after it is decoded to analog. To increase the signal level, set the Rx TLP field to a
positive number (i.e., the larger the number, the more gain is added). To decrease the signal
level, set the Rx TLP field to a negative number (i.e., the more negative the number, the more
the signal level is decreased). For example, an incoming signal at -5 dBm can be increased to
-2 dBm by setting Rx TLP to +3 dB. The Rx TLP range is -16.3 dB to +7.5 dB.

Tx TLP
The Transmit TLP setting controls the amount of gain or loss added to the voice signal coming
in from the local CPE before it is encoded to digital PCM. To increase the incoming signal
level, set the Tx TLP field to a negative value (i.e., the more negative the number, the more
gain is added). To decrease the signal level, set the Tx TLP setting to a positive number (i.e.,
the more positive the number, the more the signal level is decreased). For example, an
incoming signal at -16 dBm can be increased to 0 dBm by setting Tx TLP to -16 dB. For the
E&M 4W*8 Card, the Tx TLP range is -16.5 dB to +7.3 dB. For the E&M 4W*8 ER E&M
Card, this range is -17.5 dB to +14.5 dB.

Yep. I forgot to point that out - that tx and rx act oppositely. In the system I worked on, they had equal positive values at both ends of the ds0 essentially negating each other. You gotta love reverse Polish notation - it will send you in the wrong direction fast.

Another big problem that I have run into over the years is the lack of understanding of
the test equipment being used by the people doing the adjustments. On most audio test
equipment is a button that sets the instrument to either a bridged mode or a terminate
mode. Setting this switch wrong will result in improper level settings.

Now with that said, you need to know ahead of time if there is equipment connected to
the circuit you are trying to measure. If so, then you need to be in the bridged mode
so you don't double terminate the circuit. However, if there is no equipment connected,
then you need to be in the terminate mode so the circuit is loaded correctly to end up
with the proper levels.

Kind of confusing and not well understood by most of the techs I have observed trying
to make these adjustments. Couldn't figure out why a channel would be working fine
before a PM was done and sound like crap after the PM. Then depending on who did
the PM, the radio would sound great every time. When I started digging into the issue,
I find most of the techs have no idea what the difference was between terminated
and bridged setting on the test equipment they were using. Which position the switch
was in would depend on how it was stored in the service vehicle. The tech never
checked it before doing any audio level testing.

Anyone else run into this problem setting radio telephone circuit levels???

Jim

Yes, and I've demonstrated the difference many times to our guys.