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G8MNY > TECH 02.08.05 20:24l 104 Lines 3756 Bytes #999 (0) @ WW
BID : 56678_GB7CIP
Read: GUEST OE7FMI
Subj: Spectrum Analyser mods 88/89 4/4
Path: DB0FHN<DB0MRW<OK0PKL<OK0PPL<DB0RES<ON0AR<ZL2BAU<VK4TRS<GB7YFS<GB7CIP
Sent: 050802/1336Z @:GB7CIP.#32.GBR.EU #:56678 [Caterham] $:56678_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
FREQUENCY CONTROL
20/ The multiturn vernier centre frequency control has a 22uF bipolar capacitor
to ground (near the ICs) to remove any scratchiness.
+12vÄÄÄÄÄÄÄÄÄÄÄ¿
FREQ<ÄÄÄÄÄÄÄÄÄÂÄÄ>Tuning
10K POT ³ DC
³ === 22u
80MHz CAL ³Bipolar!
5K PRESET _³_
-12vÄÄ´<ÃÄÄÄÄÄÄÙ ////
Thermal
compensation.
21/ A multiturn preset pot on the positive rail of the control is added to
calibrate 80MHz position on the vernier scale. A diode in series gives some
temperature drift compensation. Together with the correction circuit of IC3c
fairly accurate frequency readouts are possible on the vernier scale. 0-90MHz.
IN USE
IMAGES. Other than the 0Hz line, there is only one unwanted image @ 10.7MHz, it
is at a low level & its appearance depends on the IF gain setting. It is due to
the second I.F. detector being in the same IC as the HF input section.
dB 0Hz
+70´ ³
+60´ ³
+50´ ³
+40´ ³
+30´ ³
+20´ ³
+10´ ³ Noise
0ÅÄÁÄÄÄÄÄÄ^ÄÄÄÄÄÄÄÄÄÄ Floor
10.7
VHF images are all well down due to the VHF LPF & the chip sensitivity cutting
off as well as the input filter.
OVERLOADS. These can be seen as higher levels of harmonics increasing at
greater rate than the fundamental, eg. A 10dB increase in level, causes the
fundamental to increase by 10dB (1 division) but the 2nd harmonic increases by
15 to 30dB!
dB 0Hz dB 0Hz
+70´³ +70´³ Fo
+60´³ Fo +60´³ ^ ³
+50´³ /|\ ³ +50´³ | ³ Mixer
+40´³ | ³ +40´³ | ³ Generated
+30´³ < 60dB ³ +30´³ > 60dB ³ Harmonics
+20´³ | ³ +20³³ | ³ 2Fo
+10´³ v ³ 2Fo 3Fo Noise +10´³ v ³ ³ 3Fo
0ÁÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Floor 0´ÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄ
Filtered Osc test Filtered Osc test
OVER LOADING 1st MIXER
They can also be detected as unwanted sidebands around the markers.
Other signs of overload is a raised noise floor.
dB dB
+70´ Signal +70´
+60´ ³ +60´
+50´ ³ +50´
+40´ ³ Marker +40´
+30´ ³ ³ +30´ Base Noise Floor
+20´SÄM ³ ³ S+M +20³ Raised ___
+10´Mix ³ ³ Mix +10´ ___...---"""
0´ÄÁÄÄÄÄÁÄÄÄÄÁÄÄÄÄÁÄÄÄÄ 0´ÄÄ"""
POSSIBLY OVER LOADING MIXER GROSS OVERLOAD
CLOSE IN OVERLOADS
These are much the same as above, but occur when the 2nd mixer sees 2 large
signals passing through the 1st IF filter. So if the narrowing of that filter
has been done signals will need to be closer than 1.5MHz to suffer this
problem. (eg closer than 1.5 from 0Hz reduces dynamic range & increased noise
floor)
FILTER NOISE SIDEBANDS
With large carriers are looked at close in you will see noise sidebands (phase
noise) added to the filter response, this is normal for this sort of analyser.
Lower frequency Y display filtering can mask this at the cost of peak pulse
height accuracy.
dB
+70´
+60´ /"\
+50´ ³ ³
+40´ Ý Þ
+30´ Þ Ý
+20´ Sideband ÜÝ ÞÜ Sideband
+10´ Noise _ÜÛß ßÛÜ_ Noise
0´ÄÄÄÄÄͼßßß <50KHz> ßßßÈÍÄÄÄÄÄÄÄ
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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