The AM Forum

I recently acquired a Gates BC1G transmitter already converted to 160M amateur duty. The technical manual says very little about the audio drive required other than "AUDIO INPUT: 16 dB, +-2 dB, for 100% modulation for both output powers". What audio voltage does "16 dB" specify? (not dBm, dBu, or dBv, just dB)

Bob ND9B

  I think Broadcash default is dbm..

/Dan

IIRC  dB is a rating as it is.  I'll do a little checking and get back.

Fred

A rating shown as just "dB" is referenced from 6mW equal to 0dB.  16dB is equal to 239mW.  Across a 500ohm load the voltage is 10.93 volts.  Across a 600ohm load the voltage is 12.0 volts.  The power level remains the same across any load but the voltage varies.

Use V*/R.  12*/600 = 240mW,  10.93*/500 = 239mW.

Hope this helps,

Fred

0 dB @ 600 Ohms is .77 Volts.  10 dB is 6 Volts, more or less.

The transmitter needs +16, so have a good PROFESSIONAL peak limiter with BALANCED output available or you will be chasing your tail.

Junk from some Crutchfield catalog, Ratshack, or such won't do it, as most of it can't handle that level and/or won't be in a real balanced format.

In a pinch, decent 10-Watt PA amp with a 70-Volt output will do, providing you have that peak limiter ahead of it.

That baby will easily do 100% Negative at 1100 Watts into the antenna, and clip if driven to it, and that can cost you parts failures under modulation.

73DG

Dennis,

You're beginning to worry me.  0dBm is equal to 1mW or .77v across 600ohms.  10dBm is 10mW and only 2.45v @ 600ohms.  16dBm is equal to 40mW and 4.9v @ 600ohms.

Given, you know more about a Gates BC1G than I'll ever know, but the rating was in "dB" not "dBm",  there is a difference.  A "dB" rating is based on 0dB being equal to 6mW.

All these charts are in the Radiotron Designer's Handbook.  Don't try to tell me that your dog ate your copy,  I'm not buying it.

Fred

I ate my own copy, thank you.  The dog had nothing to do with it ;)

It was directed more at having sufficient proper audio for the beast.

At least someone is paying attention to me!

73DG

ps...I've never seen two of those transmitters need the same input levels to make 100%.  I guess that's why limiters have output attenuators on 'em.

I use a behringer box ahead of a CBS limiter to drive the 600 ohm audio input of my gates BC1G..The behringer box runs at about 3/4 throttle and provides plenty of audio to give 100%+ positive modulation...The spec for the b-box says  "output level (max) balanced  +22dBu  unbalanced  +22dBu   So... dBm, dB, dBu...I'm confused again...I think we went thru this once before but I've slept since then...Steve

Steve,  All this dB stuff can be confusing.

"dBm" is related to power.  0dBm is equal to 1mW.

"dB" is also related to power  0dB is equal to 6mW.  This rating system is old and probably not used in decades.

The use of dB to reference a specific power level should not be confused when dB is used to reference a change in voltage or power levels.  Example, my amp has a 6dB power gain.  We have no way of knowing what the power output is unless we know what was the input power.  If I say that my input power is 0dB, I now know that the input power is 6mW, so my output power(6dB power gain) should be 6dB or 24mW.

dBV, dBv and dBu are related to voltage levels, usually at high impedances or even unloaded circuits.

0dBV being equal to 1.0v rms.

dBu is the same as dBV except that 0dBu is equal to 0.775v rms.

dBV, dBv and dBu do not reference power levels like "dBm" or "dB"

Hope some of this clears the confusion.

Fred

Updating and adding more confusion.

dBV and dBv are not the same.

dBv is the American voltage standard and 0dBv is equal to 0.775v rms as per Old Ma Bell.

dBV is the European voltage standard and 0dBV is equal to 1.0v rms.

dBu is the European notation for the American dBv, that's why 0dBu is also equal to 0.775v rms.

Thank you Fred...I learned on tube gear that always required a specified  audio driving voltage in VRMS  so I used  my HP VTVM for measurements..

I hope it is not used any more since it is meaningless. A dB number without a relative quantity (e.g. 1 mW in dBm) tells you nothing about the actual level, voltage, current or power of the signal.

Steve,

 "dB" power level was used, I think before dBm was used.  It was not meaningless.  A dB power level tells the same information as a dBm power level.  You just have to know that 0dB = 6mW and 0dBm = 1mW

Fred

AND, those of us (like me) that beat on this stuff daily for years, just call it a 'dB'.

Technically, it is a dBm.  Point taken.

For Bobby Dipole, it is just that feeding a BC rig is not like getting the poopy input to a Ranger or the like.

That Gates' response in comparison is like a nitrous-fed big-block compared to a lawnmower, and it's easy to blow up caps & transformers in it if done improperly.

All I'm sayin'....

73DG

Yep, like the slang use of "SWR" when technically it's VSWR.

But it's a lot easier to say swur than vees-wur, as in "That W2VW runs his vees-wur out the wazoo!"

I mean, when in a technical disgussion.

Steve,

I updated my earlier post in this thread to clear some errors in notations.

The output level from your beh. box is +22dBu.  This level output is equal to 9.75v rms, 27.6v P-P

Fred