Amplitude Modulation; Power Dbm Display Used With Am; Dynamic Range - HP 8340A Operating Manual

Model 8340A

AMPLITUDE MODULATION

The HP 8340A provides linear, de coupled amplitude modulation when internally leveled or
externally leveled with a square law detector or power meter. The input resistance is 6000
whether the AM function is on or off The sensitivity is 100% per volt +5%. This means that
+ 1.0 volt doubles the output voltage ( +6 dB), while -1.0 volt shuts the output completely
off The input accepts+ 15 V de with no damage, and is resistor-diode clamped to protect
against higher voltage transients. Most sine wave generators are calibrated in terms of RMS
voltage, so 0.707 V RMS equals 1.00 V peak The generator's output meter is accurate only if
the load impedance equals the source impedance.

POWER dBm Display used with AM

The POWER dBm display on the HP 8340A always tells actual output power. A de input to
the AM jack causes the power level to shift, and the display reflects this: + 1.0 volt causes
the display to increase 6 dB. If that much power is not available, the UNLEVELED
annunciator lights and the display shows the actual output. Inputs which reduce the ALC
level below -22 dBm will blank the display. Inputs of -1.0 volt or more negative shut off the
output and light the OVERMOD annunciator.
The POWER dBm display is filtered so that it will not flicker for AM rates above 20 Hz. The
filtering creates an average of RF voltage, which is then displayed as power in a 500 system
(dBm, 500). Therefore, modulation inputs with no de component do not cause the POWER
dBm display to shift, and this displayed number represents the power of the unmodulated
carrier. A power meter measurement of output power changes with modulation present,
increasing 1.76 dB with 100% deep sinusoidal, no de component modulation. Attempted
deep modulation at high rates causes the POWER dBm display to shift, because the AM
system cannot keep up with the input and the resultant high distortion causes a shift in
average power.

Dynamic Range

As mentioned previously with reference to Figure 3-29, the AM input is an ALC input which
does not differ in its effect from a power entry input. Therefore, the AM system is limited by
where it is operating within the ALC range. In the normal "coupled" operating mode, the
ALC will likely be set between -10 dBm and 0 dBm (see the preceding pertinent section).
Depending on frequency, the maximum available power is between + 1 dBm and +21 dBm.
The ALC is reasonably accurate down to -20 dBm, and typically is well behaved to about
-30 dBm. Expressing the desired modulation depth in dB's will let the user determine the
range over which the ALC loop is being exercised. Thirty percent AM creates excursions of
+2.3 dB to - 3.1 dB, relative to the quiescent level; 50%:
dB to - 20.0 dB. (The above assumes a modulation waveform symmetric about 0 V.)
For example: Output power= -19.0 dBm. In coupled mode this results in ATTN= -10
dB, ALC = -9.0 dBm. Ninety percent modulation depth results in an ALC range of -9.0
dBm+ 5.6 dB= -3.4 dBm maximum, -9.0 dBm - 20 dB= - 29.0 dBm minimum. This is
within the ALC limits at any frequency but the distortion may suffer due to operation below
-20 dBm. Using decoupled operation the desired output power may be set with ATTN=
-20 dB, ALC = + 1.0 dBm. Then 90% depth swings the ALC from +6.6 dBm to -19.0 dBm.
The distortion will probably be better under these conditions if +6.6 dBm is available at the
frequency of interest. At some frequencies +20 dBm is available, and setting ATTN= -30
dB, ALC = +11.0 dBm may give a further improvement, especially if attempting greater
than 90% depth. For minimum distortion the ALC should be used between -15 dBm and
+ 15 dBm, but not within 2 dB of maximum available power.
Scans by HB9HCA and HB9FSX
Operating Information
+3.5 dB to -6.0 dB; 90%: +5.6
3-105