80 Silicon chip Australia’s electronics magazine siliconchip.com.au
with 100kHz markers. As you rotate RE1 (‘TUNE’), the out-
put frequency changes and the cursor on this scale shifts
across the ‘dial’.
Pushing RE1’s knob in (the ‘STEP’ pushbutton) changes
the increments in which the frequency is adjusted with
each click as RE1 is rotated. When you push this button,
the underline below the LCD frequency display moves to
indicate the current step setting.
The Band switch (S4) selects between the two output
frequency ranges, 0-50MHz (left) and 70-120MHz (right),
while S5-S8 at the top, in combination with VR2 at right,
set the output amplitude.
The Band switch must be in the correct position for the
currently selected frequency to get the expected output
amplitude. The HPF is very effective at minimising en-
ergy from aliasing below 70MHz, so the output level can
be lower than expected by over 60dB if the incorrect se-
lection is made.
But no damage will occur as a result of an incorrect set-
ting.
While the upper range is described as 70-120MHz, tun-
ing and operation are maintained up to 150MHz, although
output levels fall significantly above 120MHz.
The maximum output of +7dBm is with S5-S8 all in the
up position and VR2 fully clockwise. For each 20dB of at-
tenuation you need, switch one of S5-S8 into the down po-
sition (it doesn’t matter which). Then for fine attenuation
adjustments, rotate VR2.
For example, if you want -30dBm, set any one of S5-S8
down (+7dBm - 20dB = -13dBm) and then VR2 should be
set quite low, to give an additional 17dB of attenuation.
(Note standard DDS amplitude rolloff impact above 30MHz
- see Fig.3 in part 1.)
The signal generator mode is selected with brief presses
of the Mode key (S2).
This selects between CW, AM, FM-NB (±1.5kHz devia-
tion), FM-WB (±3kHz deviation), FM-BC (±50kHz devia-
tion), or SCAN mode.
Pressing the Mode key again will select the initial CW
(unmodulated) mode, again along with the standard dis-
play screen.Frequency scanning mode
If the SCAN mode is selected, the display changes to
show the currently saved Start and End frequencies for
the scan, and the number of steps selected. At power-on,
this is set to 200 steps. If this is the first time after power
has been applied, the default frequency settings (starting
at 1MHz and ending at 30MHz) are shown. Otherwise, the
last used settings will be displayed.
Pressing the Scan button again allows each parameter to
be selected for adjustment.
Use the TUNE and STEP controls to set the Start and
End frequencies in turn; here, the STEP button selects the
tuning step as usual.
When the scan Steps parameter is selected with the
SCAN button, the TUNE control has no effect but pressing
the STEP button allows the number of steps to be selected
(10, 20, 50, 100, 200 or 500 per scan).
Finally, pressing SCAN again saves the selected values
and starts the scan. The display now reports SCAN instead
of the number of steps.
The scanning frequency increment is calculated by the
processor using the entered values. The scanning speed is
surprisingly fast.
Scanning may be interrupted and restarted using the
SCAN key. When stopped, the Start and End frequencies,
as well as the number of scan steps, can be adjusted again,
and the scan restarted.
To exit the scan mode, press the MODE key. This also
stops the scan and resets the signal generator to the last
scanned frequency, and CW mode.
At each stage, the output can be checked with a suitable
oscilloscope or with other RF test instruments.Performance
Typical output signals from the Generator are shown in
Figs.9-12. These were captured using a Siglent 3GHz spec-Fig.9: the CW (carrier wave, ie, unmodulated) output at
10MHz/-28dBm with a span of about 9-37MHz, selected
to include the first two harmonics. This shows the second
harmonic (20MHz) at around -40dB and the third (30MHz)
at around -47dB.
Fig.10: analysis of the AM output at 10MHz/-12dBm with a
20kHz span (ie, 9.99-10.01MHz). The 1kHz sidebands are
visible either side of the carrier, as are the 1kHz modulation
tone distortion products at ±2kHz (-21dB below the 1kHz
fundamental) and ±3kHz (-26dB below the fundamental)
indicating acceptable audio distortion levels. The
modulation depth is the industry test standard, 30%.