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G8MNY > TECH 27.01.17 12:06l 290 Lines 14675 Bytes #999 (0) @ WW
BID : 32835_GB7CIP
Read: GUEST
Subj: Spectrum Analyser mods 88-89/2
Path: IW8PGT<CX2SA<GB7CIP
Sent: 170127/1048Z @:GB7CIP.#32.GBR.EURO #:32835 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Mar 15)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
2nd IF FILTER
11/ The 2nd mixer's output has in internal pull up of 1k5 to so get 330ê source
impedance for the filter a 430ê on pin 4 determines the 2nd IF filter source
impedance. (This is not applicable to the Mark 2 with narrow filter option)
³ .-Ä-Ä-. ³ .--. If the filters are deliberately
³ Þ Ý ³ / \ mismatched then they can give
³ Ý COMMS Þ ³ | | a better analyser friendly
³ | Filter | ³ | | "rounded peak response" rather
³_./ 50kHz \._ ³_./ \._ than the flat topped ringy
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ edges of communication filters.
Correctly terminated Mis Terminated
RINGY FILTER sweep friendly filter
To find the best values for your filters use small 1k presets to source &
terminate the filters to see this effect on the display of a carrier, find the
optimum value for best IF shape & then replace the presets with nearest fixed
values.
+12VÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄ¿
=== u1 ³ IF ³
+6V _³_ ³ AMP 470ê
2nd Pin 8<Ä´ 220k ÚÄÄÄÄ´
Mixer 430ê ³ ³/ ³
602 Pin 4>ÄÁÄ¿ ÚÄÄÄÄÄÄÄÁÄÄ´ T1 ³ ÚÄÄÄÄ¿ ÚÄÄÄÂÄÄÄÄÄÄ>Pin 7 To
Pin _³_ _³_ BFX90³\e _³_ _³_ _³_ _³_ 330ê MC3356
3 ÚÄÄÄÄÄ¿ ³ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ÃÄÄÄÄÄÄ>Pin 9 Log
³ 50kHzÀÄÄÄÄÄÙ 100ê ÀÄÄÄÄÄÙ ÀÄÄÄÄÄÙ ÀÄ´ÃÄÂÄ>Pin 8 Detector
³ 10.7 ÄÄÄÂÄÄÄ Preset ÄÄÄÂÄÄÄ ÄÄÄÂÄÄÄ ===
ÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄ>Pin 19
1st filter 2nd Wide filter
Adjust the 100ê gain preset for the optimum noise floor that can just be seen.
12/ Adding an inter filter buffer stage using a single transistor T1 adds some
preset gain for setting the overall noise floor of the analyser, as well as
matching into a 2nd filter. This filter is terminated by the input load on the
detector 330ê. Two 50kHz 10.7MHz ceramic filters provide a reasonable
compromise of selectivity for sweeping 0-80MHz @ 50Hz without too much ringing
distorting & loss of the peaks levels (up to 10dB) while still looking good in
close "zoomed in" sweeps. However a 3rd filter was put in tandem with the 2nd
filter to clean up poor filter skirt rejection of my particular narrow filters
@ 7MHz!
LOG DETECTOR
13/ The detector (S meter output) uses 5 IF amps & detectors (with limiters) to
obtain log response & it is quite accurate for over 40dB range. But ignoring
the slight overload in the mixers this range can be extended on the display, by
increasing the gain calibration preset (1k preset now 2k2), & then adding a non
linear correction attenuator with diodes D1 (Schotky/Ge) & D2 (Si) to give 30%
stretch @ the highest & lowest levels where the detector has lower sensitivity.
Ge 1.0 ´Output from _.-'
MC3356 pin ÚÄ´>ÃÄ¿ Y to .9 ´Detector .-'
LOG 14>ÄÂÄÄÄ´ ÃÄÄÄÄÂÄÄÄÄ> scope .8 ´ .'
DETECTOR ³ ÃÄ10kÄ´ ³ .7 ´ .'
2n2³ 680ê ³ ³ T2 .6 ´ .' S
Video === ³ 22k \³ Timebase .5 ´ .' Correction
Filter ³ 2k2 _³_ ÃÄ<Sync/ .4 ´ _.-'
³ Y CAL \_/Si e/³ Blanking .3 ´_.-'
Pin 11>ÄÄ´ÃÄÁÄÄÄÁÄÄÂÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄ .2 ´
Lim out 1n _³_ .1 ´ Display
is RF 0v ÅÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂ>output
grounded 0 10 20 30 40 50 60 70 dB
With this correction you can get good display linearity to 70dB. Easily tested
with input attenuator & a signal generator to see equal height 10dB steps.
14/ The sensitivity is set by the Y Calibration preset to give 100mV/10dB. A
2.2nF capacitor limits the Y video bandwidth to about 10kHz (50%), but having
hardly any degradation of pulse height at the widest sweep range.
VIDEO FILTER
15/ For some applications lower video bandwidth is needed to reduce noise, I
added a 33nF switched across the Y output incorporated with the above mod, to
give about 1kHz Y bandwidth (50%).
Ä10kÄÄÄÂÄ>Y to
³ scope
o\Ä´ÃÙ
_³_ 33nF
It needs to be switchable as it causes the output to lie about the fine detail
with wide sweeps.
POWER SUPPLY
16/ The hot +12V regulator has been heatsinked, & the + rail input smoothing
capacitor increased from 680uF to 2m2. Transformer & rectifier pulse currents
wiring & layout have been kept away from the regulators as far as possible to
reduce supply hum ripple pickup.
Heatsink
4x 1N4001 +17V ÚÄÄÄÄ¿
ÚÄÄÄÂÄ´>ÃÄÂÄÄÄÄÄÄÄÂÄÄ´7812ÃÄÂÄÄÄÄÄÄ> +12V Much larger output
L >Äo/ oÄÂÄ¿ ³ _³_ _³_ 2m2 ³+ ÀÄÂÄÄÙ+³330u 200mA caps have been
100k )º( /_\ /_\ 25v=== ³ ===16v used to reduce
240V ³ )º(___ ³ ___ ³ ______³____³____³_____\ 0V the last remnants
NEON )º( ³ ³ 680u³+ ³ ³+ / of hum & noise.
³ )º( ³ ³ 25v=== ³ ===330u This is most
N >ÄÄÄÄÄÄÁÄÙ ³ ³ ÀÄÄÄÄÄ)ÄÄÄÄÄÄ´ ÚÄÁÄÄ¿ ³ 16v important
³ ÀÄÄÄÄÄÄÄÄÂÁÄÄ´<ÃÄÁÄÄÄÄ´7912ÃÄÁÄÄÄÄÄ> -12V for close in
E >ÄÄÂÄÄÄÄÄÄÄÙ 14-0-14 ³ -19V ÀÄÄÄÄÙ 20mA stability.
ÀÄ> 0V 0.3A ÀÄ>50Hz
The mains transformer has also been varnished to reduce acoustic hum & an outer
copper short circuit added to reduce magnetic fields that can affect the 2nd
osc stability. The other 2 low power regulators +6V for 2nd osc, & +5V for Log
amp, are placed near those circuits for best noise/voltage error rejection.
17/ A 50Hz synchronisation line is provided for ramp timebase locking. This is
important for close "zoomed in" stability of the sweep.
RAMP GENERATOR
In the simple mark 1 design, it uses 4 operational amplifiers IC3 (e.g. TL084)
that run on the ñ12V. The original circuit produced a symmetrical 500Hz ramp up
& down oscillator which was far too fast for wide sweeps & half the time was
wasted during the flyback. Mod 18/ solves this.
18/ IC3a forms a 50Hz ramp oscillator with the 100k & 12k in parallel during
flyback due to the diode D3, & a 1uF timing capacitor to give close to mains
frequency. Then a small injection of 50Hz from the mains transformer alters the
flyback time (D3, 12k & 1uF) to cause lock up to mains frequency. This method
ensures constant sweep MHz rate & the mains lock ensure a stable display even
with some sweep hum present when zoomed close in.
Ä¿ÚÄÄÄ¿
50Hz 12VAC >ÄÄ100kÄÂÄ12kÄÂÄÄÂÄÄÄÄÄÄÄ¿ ÀÙ ÀÙ _
from bridge _³_ ³ 100k ³ Ú¿ _³ \____
MAINS D3 /_\ 100k ³ ³ ÄÙÀÄÄ
LOCKED /ÃÄÄÄÄÄÄÄÄ´ ³ ÃÄÄ´\ ³ - ÚÄÄ´>ÃÄÂÄ22kÄ>Y Blanking
50Hz <ÄÄÄÄÄÂÄÄÄÄ<'ñ³IC3b ³ ³ ³ ³ñ`>ÄÁÄÄ´\ ³ ³ Transistor
RAMP,. ³ n47 `\ÃÄÂÄÄ¿ ÃÄÄÄÄÄÁÄÄ)ÄÄ´/' +³ `>ÄÄÁÄÄ2k2Ä´ T2
,/ ³ ÃÄ´ÃÄÄÄÄÄÄ´ ³ ³ ³ IC3a ÚÄÄ´/' ³
,/ ³,/ ÀÄÄÄ10kÄÄÄÙ 1k2 ===1u 10k ³ IC3d u1===
' ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄ
Buffer x9 ,/'³,/'³ 50Hz Ramp Osc Sweep Settle Delay
19/ IC3d buffers &
inverts the banking dB 0Hz /|\
pulse & it is +70´ ³ | | + 0.9V
lengthened with CR +60´ ³ Sweep |
& a diode before it +50´ ³ Centre 100mV
drives blanking +40´ ³ | /10dB
transistor T2. This +30´ º |
eliminates any sweep +20´ º |
folding due to VHF osc +10´ º Noise Floor \|/
sweep settling delay 0´ ÚßÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ + 0.2V
with RF sweep filtering SYNCS³ SWEEP ^ ³
to be masked. And the ÄÄÄÙ | ÀÄÄÄÙ - 0V
banking also gives Scope^ Scope
the scope a 0V sync Trigger Flyback
pulse to lock to. < - - - - 20mS - - - - ->
20/ IC3c is the sweep correction amplifier, this amplifies the selected sweep
width together with the centre frequency DC, then corrects for VHF oscillator
varicap frequency control non linearity by pre-distorting the ramp waveform
with 5 gain changes using 4 diodes, D4-D6 & ZD2 zener.
-12V>Ä3k3ÄÄÄÂÄÄ2k2ÄÄ<+12V MHz Tune
10 Turn 390k 240´Ramp _
Tune<Ä68k¿ R1 230´Input _..--''~
Pot ÃÄÄ27kÄÄÂÄ´<ÃÄ´ 220´ Five _.--'~ R4
³ 27k _³_ 210´ Slopes _.-' R3
Ramp ³ ÚÄ´\ ³ /_\ To VHF 200´ .-'
³ ³ ³ ³ñ`>ÁÄÄÄÄÄ)ÄÄÄÄÄ>Osc 190´ .-' R2
Sweep ÃÄ)Ä´/'IC3c ³ Varicap 180´ .'
Pot<Ä68kÙ ³ ³ 2.7V 170´ .' R1
10k ³ ÃÄ´>ÃÄÄÂÄ´<ÃÄ¿ 160´ ;
³ ³ R2 47k Ù 15k 4k7 150´: Varicap Volts (ref to +12V)
ÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÁ 140ÅÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂ
R3 R4 +12 9 6 3 0 -3 -6 -9 -12
The R1-4 values used for the gain corrections are set up using the marker to
give even spacings on the display.
0Hz
³
³ . Even spaced 5MHz markers
³ ³ . ³ | | . . .
º ³ ³ ³ ³ ³ | ³ | ³ | ³ | ³ | ³ | ³
º ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ 90
_º_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_
5 15 25 35 45 55 65 75 85 MHz
FREQUENCY CONTROL
21/
+12VÄÄÄÄÄÄÄÄÄÄÄ¿
FREQ<ÄÄÄÄÄÄÄÄÂÄÄ>Tuning The multiturn vernier
10k POT ³ DC centre frequency control
³ === 22uF has a 22uF bipolar
80MHz CAL ³Bipolar! capacitor to ground
5k PRESET _³_ (or elect to +12V near the ICs)
-12VÄÄ´<ÃÄÄÄÄÄÄÙ //// to remove any pot scratchiness.
Thermal
compensation.
22/ 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.
I N U S E
IMAGES
Other than the 0 Hz 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 IF detector being in the same IC as the RF input section!
dB 0Hz
+70´ ³
+60´ ³ VHF images are all well
+50´ ³ down due to the VHF LPF
+40´ º & the chip sensitivity
+30´ º cutting off as well as
+20´ º the input filter & double
+10´ º Noise screened box.
0ÅÄßÄÄÄÄÄÄ^ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Floor
10.7MHz
OVERLOADS
These can be seen as higher levels of harmonics increasing at a greater rate
than the fundamental. e.g. 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´º <60dB³ +30´º >60dB³ Harmonics
+20´º | º +20³º | º 2Fo
+10´º v º 2Fo 3Fo Noise +10´º v º ³ 3Fo
0ÁßÄÄÄÄÄÄÐÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Floor 0´ßÄÄÄÄÄÄßÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄ
Filtered Osc test Filtered Osc test
OVER LOADING 1st MIXER
They can also be detected as unwanted sidebands around the markers too.
dB dB
+70´ Signal +70´
+60´ ³ +60´
+50´ ³ +50´
+40´ ³ Marker +40´
+30´ ³ ³ +30´ Base Noise Floor
+20´ SÄM º ³ S+M +20³ Raised __
+10´ Mix º ³ Mix +10´ __..--""~~
0´ÄÄÁÄÄÄÄßÄÄÄÄÐÄÄÄÄÁÄÄÄÄ 0´ÄÄ""~~
POSSIBLY OVER LOADING MIXER GROSS OVERLOAD
Other signs of overload is a raised noise floor.
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, strong signals will need to be closer than 1 MHz to suffer this
problem. (e.g. using the analyser closer than 1 MHz from 0 Hz reduces dynamic
range due to the increased noise floor from its' own 2 oscillators)
FILTER NOISE SIDEBANDS
dB With large carriers are looked
+70´ at close in, you will see noise
+60´ /~\ sidebands (phase noise) added
+50´ ³ ³ to the filter response, this is
+40´ Ý Þ normal for this sort of analyser.
+30´ Þ Ý
+20´ Sideband ÜÝ ÞÜ Sideband Lower frequency Y display filtering
+10´ Noise _ÜÛß ßÛÜ_ Noise can mask this, but at the cost of
0´ÄÄÄÄÄͼßßß <50kHz> ßßßÈÍÄÄÄÄÄÄÄ peak pulse height accuracy.
SWEEP NOISE
Some of this phase noise can be noisy sweep amps, as the S/N needed on the VHF
osc will be >120dB, e.g. 24V max sweep & < 24uV of noise! I have used active
sweep filtering on some analysers to overcome this failing where the sweep rate
is low & a CR filter after the last opamp does reduce the HF noise sent to the
oscillator.
ÚÄ´>ÃÄ¿ The diode direction (depends on circuit)
³\ ³ ³ ensures the flyback charges up the cap
OP ³ `>ÄÁÄÄRÄÄÁÂÄÄÄÄ>VHF OSC voltage quickly, so there is little cramping
amp ³/' === C at the start of the sweep.
ÄÄÁÄÄ e.g. Xc = R (-3dB) @ 10x sweep freq.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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