About the article

DDS RF Signal Generator

Frequency range: 50 Hz to over 70 MHz

DDS RF Signal Generator
Radio amateurs and RF engineers in general will welcome this design. This contemporary RF signal generator has many bells and whistles and is just the ticket for testing HF/VHF receivers, aligning filters, IF amplifiers and AM/FM demodulators. The instrument can even act as a source for very low frequencies starting at 50 Hz.While the combination of an ordinary multimeter and a rudimentary signal tracer is perfectly adequate for many ‘kitchen table’ hobbyists to get their homebrew circuits working, more sophisticated equipment is typically required by those interested in radio and higher frequency circuits in general. In particular, the more complex designs in the RF realms normally require accurate adjustment, which in practice translates into a decent RF signal generator with an internal or external modulation option available.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Extra info / Update
Constructors are advised that a new version of the controller firmware is now available (ref. 020299-41). The update solves the problem of the software crashing when the ‘B’ key is pressed to switch between dBm and V. For a free update, return your controller chip with the Elektor ESS sticker on it to Readers Services, att. Mr. J. Visser, Software Service. In some cases, the display reading does not change when the rotary encoder is turned quickly. The problem is solved by connecting a 10-nF capacitor between pin 13 of the microcontroller and ground. The values of R7 and R8 should be interchanged in the schematic and the parts list. The correct descriptions are: R7 = 86.6 Ohm; R8 = 43.2 Ohm.

Pin 6 of IC3 is erroneously connected to the +9 V supply rail. The relevant IC input is however designed to handle logic levels with a maximum High level of +5 V. Although the IC will not take damage from the +9 V level applied, it is recommended to cut the relevant connection and wire pin 6 to the +5 V supply rail.
Component list
Signal generator board (020299-1)
R1,R2 = 49?9
R3 = 6k?8
R4,R9 = 53?6
R5 = 12k?
R6 = 10k?
R7 = 43?2
R8 = 86?6
R10-R13,R16-R19,R22-R25 = 105?
R14,R20,R26 = 1k?69
R15,R21,R27 = 2k?0
R28 = 1k?
R29 = 560?
R30 = 100k?
R31 = 150k?
C34,C38,C39 = 100nF, 5mm lead pitch
C4,C6,C7,C9,C25-C29,C31 = 100nF,
SMD shape 0805
C11,C14,C16,C19 = 18pF
C12,C13,C17,C18 = 68pF
C15,C20 = 5pF6
C21 = 10pF
C22 = 40pF trimmer
C23 = 22pF
C24 = 220pF
C35,C36 = 22µF 10V radial
C37 = 47µF 10V radial
C40 = 10nF
L1-L5,L16,L18 = 4µH7
L6,L11 = 120nH
L7,L9,L10,L12,L14,L15 = 150nH
L8,L13 = 220nH
L17 = 1µH2
D1,D2,D3 = 1N4148
T1-T4 = BS170
IC1 = 74HCU04
IC2 = AD9851BRS
IC3 = AD8321AR
IC4 = 7805
IC5 = 7809
K1 = 16-way boxheader (2x8)
K2 = 10-way boxheader (2x5)
X1 = 10MHz quartz crystal (series resonance. CL 32pF) or 30MHz DIL14 oscillator module
Re1,Re2,Re3 = TQ2-9V or TQ2-12V 3 wire links
Enclosure, tin sheet dim. 160x48x25 mm
PCB, order code 020299-1
Control/power supply board
R1 = 4-way 22k? SIL array
R2 = 82? 5W
R3 = 10k?
R4 = 330?
P1 = 10k? preset
C1,C2,C5,C7,C8,C10 = 100nF, 5mm lead pitch
C3,C4 = 33pF
C6 = 270pF
C9 = 4700µF 25V radial
B1 = B80C1500, rectangular case (80V piv, 1.5 A)
IC1 = MAX7400CPA
IC2 = AT90S8515 8PC, programmed, order code 020299-41
IC3 = 7805
K1,K3 = 10-way boxheader (2x5)
K2,K4 = 16-way boxheader (2x8)
K5 = 2-way PCB terminal block, lead pitch 7.5mm
S1 = rotary encoder, Bourns ECW1J or ddm427 (Conrad Electronics)
X1 = 8MHz quartz crystal, parallel resonance, CL 32pF
TR1 = 12V/4.8VA mains transformer, e.g., Gerth 1x12V/400 mA
F1 = fuse, 32 mAT with PCB mount fuse holder
5 wire links
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